diff --git a/package.json b/package.json
index 8fffac7..ca30b40 100644
--- a/package.json
+++ b/package.json
@@ -7,6 +7,7 @@
 		"build": "strapi build --debug",
 		"strapi": "strapi",
 		"lint:fix": "prettier --write \"{src,types,config}/**/*.{js,jsx,ts,tsx}\"",
+		"test": "node --experimental-strip-types --import ./tests/unit/support/register.mjs --test 'tests/unit/**/*.test.ts'",
 		"generate:types": "strapi ts:generate-types",
 		"import": "npm run strapi import -- -f database/export.tar.gz --force",
 		"export": "npm run strapi export -- --no-encrypt --file database/export_$(date +'%Y%m%d%H%M%S')",
diff --git a/tests/unit/log-format.test.ts b/tests/unit/log-format.test.ts
new file mode 100644
index 0000000..5265264
--- /dev/null
+++ b/tests/unit/log-format.test.ts
@@ -0,0 +1,80 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { formatLogLine } from "../../src/logging/format.ts";
+
+const TIMESTAMP = "2026-10-08 21:29:40.426";
+const CREDENTIALS = "RFVNTVlfQ0xJRU5UX0lEOkRVTU1ZX0NMSUVOVF9TRUNSRVQ="; // base64 of the dummy "DUMMY_CLIENT_ID:DUMMY_CLIENT_SECRET"
+
+// Characterization: what the console line looks like today.
+
+test("renders timestamp, level and message, with a trailing space when there are no other fields", () => {
+	assert.equal(
+		formatLogLine({ timestamp: TIMESTAMP, level: "info", message: "app:i:paypal-api: hello" }),
+		"2026-10-08 21:29:40.426Z info: app:i:paypal-api: hello "
+	);
+});
+
+test("appends the other fields as JSON indented by two spaces", () => {
+	assert.equal(
+		formatLogLine({
+			timestamp: TIMESTAMP,
+			level: "debug",
+			message: "app:d:order-service: Calculating totals",
+			productsTotal: 17.9,
+			deliveryPrice: 4.95
+		}),
+		'2026-10-08 21:29:40.426Z debug: app:d:order-service: Calculating totals {\n  "productsTotal": 17.9,\n  "deliveryPrice": 4.95\n}'
+	);
+});
+
+test("ignores the symbol keys winston adds", () => {
+	const info = { timestamp: TIMESTAMP, level: "info", message: "hello", [Symbol.for("level")]: "info", [Symbol.for("splat")]: [] };
+
+	assert.equal(formatLogLine(info), "2026-10-08 21:29:40.426Z info: hello ");
+});
+
+// Known defects: these assert the correct behaviour and fail today, which the todo marks as expected.
+
+test(
+	"does not throw on a circular reference",
+	{ todo: "JSON.stringify throws on a circular reference — https://git.librete.ch/libretech/mp/issues/67" },
+	() => {
+		const request: Record<string, unknown> = { method: "POST", path: "/v1/oauth2/token" };
+		request.self = request;
+
+		assert.doesNotThrow(() => formatLogLine({ timestamp: TIMESTAMP, level: "error", message: "app:e:paypal-api", request }));
+	}
+);
+
+test(
+	"does not write the Authorization header of an axios-like error into the line",
+	{ todo: "the PayPal client credentials are logged — https://git.librete.ch/libretech/mp/issues/67" },
+	() => {
+		const error = Object.assign(new Error("getaddrinfo EAI_AGAIN api-m.paypal.com"), {
+			name: "AxiosError",
+			code: "EAI_AGAIN",
+			config: { method: "post", url: "https://api-m.paypal.com/v1/oauth2/token", headers: { Authorization: `Basic ${CREDENTIALS}` } }
+		});
+		// What strapi.log.error(error) hands this formatter: Strapi's logErrors format spreads the error and appends its stack.
+		const line = formatLogLine({ ...error, message: `${error.message}\n${error.stack}`, level: "error", timestamp: TIMESTAMP });
+
+		assert.ok(!line.includes(CREDENTIALS), line);
+	}
+);
+
+test(
+	"renders an Error with its message instead of {}",
+	{ todo: "an Error is rendered as {} — https://git.librete.ch/libretech/mp/issues/67" },
+	() => {
+		const error = new Error("getaddrinfo EAI_AGAIN api-m.paypal.com");
+		// strapi.log.error("…", { error }), as in the order controller's capturePayment
+		const line = formatLogLine({
+			timestamp: TIMESTAMP,
+			level: "error",
+			message: "app:e:order-controller: Error capturing payment for order x",
+			error
+		});
+
+		assert.ok(line.includes("getaddrinfo EAI_AGAIN api-m.paypal.com"), line);
+	}
+);
diff --git a/tests/unit/paypal-order.test.ts b/tests/unit/paypal-order.test.ts
new file mode 100644
index 0000000..104baea
--- /dev/null
+++ b/tests/unit/paypal-order.test.ts
@@ -0,0 +1,234 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { buildPayPalOrderItems, buildPayPalOrderRequest } from "../../src/checkout/paypal-order.ts";
+import { calculateTotalProductPrice } from "../../src/checkout/pricing.ts";
+import { vatDecimal, vatIncludedDecimal } from "../../src/checkout/vat.ts";
+
+const RETURN_URL = "https://shop.example/checkout/3";
+const UUID = "11111111-2222-4333-8444-555555555555";
+
+const notebook = { id: 101, name: "Notizbuch A5 Punktraster", cover: { price: 14.9 }, pages: { price: 3 }, ruling: { price: 0 } };
+
+// Shaped like the order the checkout controller passes to createSessionOrThrow (strapi.db.query findOne with orderDefaultParams),
+// with the totals the order service computes: subtotal 17.90, total 17.90 + 4.95 = 22.85, VAT round(17.90 / 1.19 * 0.19) = 2.86.
+const validOrder = (overrides = {}) => ({
+	id: 4711,
+	uuid: UUID,
+	email: "erika@example.org",
+	subtotal: 17.9,
+	total: 22.85,
+	VAT: 2.86,
+	delivery: { id: 1, name: "DHL", price: 4.95 },
+	payment: null,
+	invoiceNumber: "R-12-4711",
+	cart: [{ id: 1, count: 1, product: notebook }],
+	deliveryAddress: "Erika Mustermann\nMusterstraße 1\n70190 Stuttgart",
+	...overrides
+});
+
+// A cart of [price, count] lines with the totals the order service computes in update() (src/api/order/services/order.ts:151-173).
+const orderFor = (lines: [number, number][], deliveryPrice: number | null) => {
+	const cart = lines.map(([price, count], index) => ({
+		id: index + 1,
+		count,
+		product: { id: 201 + index, name: `Produkt ${index + 1}`, cover: { price } }
+	}));
+	const subtotal = cart.reduce((v, p) => v + (calculateTotalProductPrice(p.product) ?? 0) * p.count, 0);
+	return validOrder({
+		cart,
+		subtotal,
+		total: Math.round((subtotal + (deliveryPrice ?? 0)) * 100) / 100,
+		VAT: Math.round((subtotal / vatIncludedDecimal) * vatDecimal * 100) / 100,
+		delivery: deliveryPrice === null ? null : { id: 1, name: "DHL", price: deliveryPrice }
+	});
+};
+
+// Both halves in the order createSessionOrThrow runs them: what it would send to PayPal, or what it would throw.
+const buildSession = (order) => {
+	buildPayPalOrderItems(order);
+	return buildPayPalOrderRequest(RETURN_URL, order);
+};
+
+const cents = (money) => (money ? Math.round(Number(money.value) * 100) : 0);
+
+// PayPal rejects an order unless amount = item_total + tax_total + shipping + handling + insurance - shipping_discount - discount.
+const breakdownCents = (breakdown) =>
+	cents(breakdown.itemTotal) +
+	cents(breakdown.taxTotal) +
+	cents(breakdown.shipping) +
+	cents(breakdown.handling) +
+	cents(breakdown.insurance) -
+	cents(breakdown.shippingDiscount) -
+	cents(breakdown.discount);
+
+const isClearError = (pattern: RegExp) => (error: unknown) => error instanceof Error && !(error instanceof TypeError) && pattern.test(error.message);
+
+const assertNoTypeError = (build: () => unknown) => {
+	try {
+		build();
+	} catch (error) {
+		assert.ok(!(error instanceof TypeError), `threw ${error}`);
+	}
+};
+
+// Characterization: what the builders do today.
+
+test("builds the PayPal order request for a valid order", () => {
+	const expected = {
+		intent: "CAPTURE",
+		purchaseUnits: [
+			{
+				referenceId: UUID,
+				amount: {
+					currencyCode: "EUR",
+					value: "22.85",
+					breakdown: {
+						itemTotal: { currencyCode: "EUR", value: "15.04" },
+						taxTotal: { currencyCode: "EUR", value: "2.86" },
+						shipping: { currencyCode: "EUR", value: "4.95" }
+					}
+				},
+				shipping: {
+					name: { fullName: "Erika Mustermann" },
+					address: { addressLine1: "Musterstraße 1", postalCode: "70190", adminArea2: "Stuttgart", countryCode: "DE" }
+				},
+				customId: UUID,
+				invoiceId: "R-12-4711"
+			}
+		],
+		applicationContext: { returnUrl: RETURN_URL, cancelUrl: RETURN_URL, shippingPreference: "SET_PROVIDED_ADDRESS" }
+	};
+	const request = buildPayPalOrderRequest(RETURN_URL, validOrder());
+
+	assert.deepEqual(request, expected);
+	// The key order is visible in the "Creating PayPal order" log line.
+	assert.equal(JSON.stringify(request), JSON.stringify(expected));
+});
+
+test("sends no shipping amount when the order has no delivery method", () => {
+	assert.deepEqual(buildPayPalOrderRequest(RETURN_URL, validOrder({ delivery: null, total: 17.9 })).purchaseUnits[0].amount, {
+		currencyCode: "EUR",
+		value: "17.90",
+		breakdown: {
+			itemTotal: { currencyCode: "EUR", value: "15.04" },
+			taxTotal: { currencyCode: "EUR", value: "2.86" },
+			shipping: undefined
+		}
+	});
+});
+
+test("rejects an order without a total before computing the items", () => {
+	for (const total of [0, null, undefined]) {
+		assert.throws(() => buildPayPalOrderItems(validOrder({ total })), { name: "Error", message: "Cart is empty or has no total" });
+	}
+});
+
+test("computes one item per cart line, which is not sent to PayPal", () => {
+	const order = validOrder({
+		cart: [
+			{ id: 1, count: 1, product: notebook },
+			{ id: 2, count: 3, product: { id: 102, name: "Journaling Booklet", cover: { price: 12.5 } } }
+		]
+	});
+
+	assert.deepEqual(buildPayPalOrderItems(order), [
+		{
+			name: "Notizbuch A5 Punktraster",
+			unitAmount: { currencyCode: "EUR", value: "15.04" },
+			tax: { currencyCode: "EUR", value: "2.86" },
+			quantity: "1"
+		},
+		{
+			name: "Journaling Booklet",
+			unitAmount: { currencyCode: "EUR", value: "10.50" },
+			tax: { currencyCode: "EUR", value: "2.00" },
+			quantity: "3"
+		}
+	]);
+});
+
+// Known defects: these assert the correct behaviour and fail today, which the todo marks as expected.
+
+test(
+	"D1: a missing delivery address is rejected with a clear validation error",
+	{ todo: "null deliveryAddress throws a TypeError — https://git.librete.ch/libretech/mp/issues/67" },
+	() => {
+		assert.throws(() => buildSession(validOrder({ deliveryAddress: null })), isClearError(/address/i));
+	}
+);
+
+test(
+	"D2: a two-line delivery address does not throw a TypeError",
+	{ todo: "a two-line address throws a TypeError — https://git.librete.ch/libretech/mp/issues/67" },
+	() => {
+		assertNoTypeError(() => buildSession(validOrder({ deliveryAddress: "Erika Mustermann\nMusterstraße 1" })));
+	}
+);
+
+test(
+	"D3: a pickup whose delivery price is null is sent with shipping 0.00",
+	{ todo: "delivery price null throws a TypeError — https://git.librete.ch/libretech/mp/issues/67" },
+	() => {
+		const order = validOrder({ delivery: { id: 2, name: "Abholung", price: null }, total: 17.9 });
+
+		assert.deepEqual(buildSession(order).purchaseUnits[0].amount.breakdown.shipping, { currencyCode: "EUR", value: "0.00" });
+	}
+);
+
+test(
+	"D4: a cart line whose product was deleted is rejected with a clear error",
+	{ todo: "a deleted product throws a TypeError — https://git.librete.ch/libretech/mp/issues/67" },
+	() => {
+		assert.throws(() => buildSession(validOrder({ cart: [{ id: 1, count: 1, product: null }] })), isClearError(/product/i));
+	}
+);
+
+test(
+	"D5: with a payment surcharge the breakdown still adds up to the amount",
+	{ todo: "the surcharge is in the amount but not in the breakdown — https://git.librete.ch/libretech/mp/issues/67" },
+	() => {
+		// total = 17.90 + 4.95 shipping + 1.50 surcharge
+		const { amount } = buildSession(validOrder({ payment: { id: 1, name: "PayPal", price: 1.5 }, total: 24.35 })).purchaseUnits[0];
+
+		assert.equal(amount.value, "24.35");
+		assert.equal(breakdownCents(amount.breakdown), cents(amount));
+	}
+);
+
+// Not a todo: this holds today for every whole-cent subtotal (Strapi stores prices as decimal(10,2)). It guards the fixes above.
+test("D6: the breakdown adds up to the amount to the cent, also for the subtotals whose net is closest to half a cent", () => {
+	const carts: { lines: [number, number][]; delivery: number | null }[] = [
+		{ lines: [[17.9, 1]], delivery: 4.95 },
+		// subtotal 36.10000000000001 in floating point
+		{
+			lines: [
+				[4.15, 7],
+				[2.35, 3]
+			],
+			delivery: 4.95
+		},
+		// 29.03: net 24.394958, as close to a half cent as a whole-cent subtotal gets
+		{
+			lines: [
+				[9.99, 2],
+				[9.05, 1]
+			],
+			delivery: 4.95
+		},
+		// 41.18: net 34.605042, the same from above; no delivery method
+		{ lines: [[20.59, 2]], delivery: null },
+		{ lines: [[5.23, 1]], delivery: 4.95 },
+		{ lines: [[5.48, 1]], delivery: 4.95 }
+	];
+	for (let cent = 1; cent <= 30000; cent++) {
+		carts.push({ lines: [[cent / 100, 1]], delivery: 4.95 }, { lines: [[cent / 100, 1]], delivery: null });
+	}
+
+	const mismatches = carts.filter(({ lines, delivery }) => {
+		const { amount } = buildSession(orderFor(lines, delivery)).purchaseUnits[0];
+		return breakdownCents(amount.breakdown) !== cents(amount);
+	});
+
+	// Compare the count, not the array: diffing tens of thousands of entries against [] takes many GB of memory.
+	assert.equal(mismatches.length, 0, `${mismatches.length} carts do not add up, e.g. ${JSON.stringify(mismatches.slice(0, 3))}`);
+});
diff --git a/tests/unit/pricing.test.ts b/tests/unit/pricing.test.ts
new file mode 100644
index 0000000..557200b
--- /dev/null
+++ b/tests/unit/pricing.test.ts
@@ -0,0 +1,26 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { calculateTotalProductPrice } from "../../src/checkout/pricing.ts";
+
+// Characterization: what calculateTotalProductPrice returns today.
+
+test("adds up the cover, pages and ruling prices", () => {
+	assert.equal(calculateTotalProductPrice({ cover: { price: 14.9 }, pages: { price: 3 }, ruling: { price: 0.5 } }), 18.4);
+});
+
+test("prices a standalone product from its cover alone", () => {
+	assert.equal(calculateTotalProductPrice({ cover: { price: 9.5 } }), 9.5);
+});
+
+test("counts a missing component or price as 0", () => {
+	assert.equal(calculateTotalProductPrice({ cover: null, pages: { price: null }, ruling: undefined }), 0);
+});
+
+test("returns 0 for a missing product instead of throwing", () => {
+	assert.equal(calculateTotalProductPrice(null), 0);
+	assert.equal(calculateTotalProductPrice(undefined), 0);
+});
+
+test("does not round the sum", () => {
+	assert.equal(calculateTotalProductPrice({ cover: { price: 0.1 }, pages: { price: 0.2 } }), 0.30000000000000004);
+});
diff --git a/tests/unit/support/register.mjs b/tests/unit/support/register.mjs
new file mode 100644
index 0000000..b221a70
--- /dev/null
+++ b/tests/unit/support/register.mjs
@@ -0,0 +1,6 @@
+// Loaded by `npm test` through `node --import`. Node runs the .ts sources directly (type stripping), but unlike tsc it does not
+// resolve extensionless relative imports such as `import { vatIncludedDecimal } from "./vat"` in src/checkout/paypal-order.ts.
+// The hook below retries such an import from a .ts file with ".ts" appended. Test files import sources with explicit .ts paths.
+import { register } from "node:module";
+
+register("./resolve-ts.mjs", import.meta.url);
diff --git a/tests/unit/support/resolve-ts.mjs b/tests/unit/support/resolve-ts.mjs
new file mode 100644
index 0000000..ede799e
--- /dev/null
+++ b/tests/unit/support/resolve-ts.mjs
@@ -0,0 +1,14 @@
+// Module resolve hook, registered by ./register.mjs: `./vat` imported from a .ts file resolves to `./vat.ts`.
+export async function resolve(specifier, context, nextResolve) {
+	try {
+		return await nextResolve(specifier, context);
+	} catch (error) {
+		const relative = specifier.startsWith("./") || specifier.startsWith("../");
+		if (error?.code !== "ERR_MODULE_NOT_FOUND" || !relative || !context.parentURL?.endsWith(".ts")) throw error;
+		try {
+			return await nextResolve(`${specifier}.ts`, context);
+		} catch {
+			throw error;
+		}
+	}
+}