diff --git a/lib/coinpay.ts b/lib/coinpay.ts
index abc5fad..94caee9 100644
--- a/lib/coinpay.ts
+++ b/lib/coinpay.ts
@@ -33,6 +33,21 @@ export function payUrl(id: string): string {
 
 export type CreatedPayment = { id: string; status: string; payUrl: string };
 
+const SETUP_AMOUNT_RE = /^(?:\d+|\d{1,3}(?:,\d{3})+)(?:\.\d{1,2})?$/;
+
+export function setupAmountUsd(value: unknown = SETUP_FEE_USD): number {
+  if (typeof value === "number") {
+    if (!Number.isFinite(value) || value <= 0) throw new Error("setup payment amount must be positive");
+    return Math.round(value * 100) / 100;
+  }
+
+  const text = String(value ?? "").trim().replace(/\s+/g, "");
+  if (!SETUP_AMOUNT_RE.test(text)) throw new Error("setup payment amount must be a positive dollar amount");
+  const amount = Number(text.replace(/,/g, ""));
+  if (!Number.isFinite(amount) || amount <= 0) throw new Error("setup payment amount must be positive");
+  return amount;
+}
+
 export async function createSetupPayment(opts: {
   accountId: string;
   email: string;
@@ -44,7 +59,7 @@ export async function createSetupPayment(opts: {
   // module is imported even when CoinPay isn't configured (offline dev mode).
   const coinpay = createCoinPayClient({ apiKey: API_KEY, baseUrl: ISSUER });
   const { paymentId, payment } = await coinpay.createCheckout({
-    amountUsd: Number(opts.amount || SETUP_FEE_USD),
+    amountUsd: setupAmountUsd(opts.amount || SETUP_FEE_USD),
     currency: PAY_CHAIN.toLowerCase(),
     paymentMethod: "crypto",
     description: `moshcoding account setup — ${opts.domain}`,
diff --git a/tests/coinpay-amount.test.mjs b/tests/coinpay-amount.test.mjs
new file mode 100644
index 0000000..f1bc97e
--- /dev/null
+++ b/tests/coinpay-amount.test.mjs
@@ -0,0 +1,19 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { setupAmountUsd } from "../lib/coinpay.ts";
+
+test("setupAmountUsd accepts ordinary positive dollar amounts", () => {
+  assert.equal(setupAmountUsd("1.00"), 1);
+  assert.equal(setupAmountUsd(" 1,234.56 "), 1234.56);
+  assert.equal(setupAmountUsd(2.345), 2.35);
+});
+
+test("setupAmountUsd rejects malformed or unsafe payment amounts", () => {
+  assert.throws(() => setupAmountUsd(""), /positive dollar amount/);
+  assert.throws(() => setupAmountUsd("1e2"), /positive dollar amount/);
+  assert.throws(() => setupAmountUsd("0x10"), /positive dollar amount/);
+  assert.throws(() => setupAmountUsd("-1"), /positive dollar amount/);
+  assert.throws(() => setupAmountUsd(Number.NaN), /positive/);
+  assert.throws(() => setupAmountUsd(Number.POSITIVE_INFINITY), /positive/);
+});