Sell digital goods for crypto

CoinsFlow is a crypto payment gateway for stores that sell things delivered over the internet: keys, accounts, downloads, game items. Your store makes an invoice, the buyer pays it in Litecoin, USDT, Bitcoin or another coin we take, and a signed webhook tells your store to hand the product over. Small payments are fine: the fee is 0.5% of each one, and nothing else.

How it works

1. A buyer checks out. Your store makes an invoice with POST /v1/invoices: the price in dollars, and the product and the customer in metadata, which the buyer never sees.
2. The buyer is sent to the invoice's paymentUrl. With no coin named, they pick one there: Litecoin, Bitcoin, Ether, BNB, USDT or USDC. The page shows the address, the exact amount, a QR code and a countdown.
3. When it is paid, invoice.paid reaches your webhook: on Litecoin at the first confirmation, about 2.5 minutes; with USDT on BNB Smart Chain in about 15 seconds.
4. Your store delivers, once per invoice: it issues the key, unlocks the download, or adds the item to the buyer's account.
5. The money is in your balance. Withdraw it to your own wallet, by hand or automatically.

A $6 licence key, in numbers

The invoice

POST /v1/invoices
{
  "priceAmount": "6.00",
  "orderId": "order-8812",
  "description": "Editor Pro, 1-year licence",
  "metadata": { "sku": "editor-pro-1y", "customer": "c_19f3" }
}

Send an Idempotency-Key with it, such as your order id: a retried request gets the same invoice back instead of making a second one. metadata comes back in every webhook. The amount in the coin follows the dollar price once an hour until it is paid, so send the buyer the link rather than a number. Every field is in the API docs.

The webhook delivers, once

import express from "express";

// Your products, by the sku the store put in metadata. Nothing else is ever delivered.
const PRODUCTS = {
  "editor-pro-1y": () => issueKey("editor-pro", 365), // your licence server
  "pack-starter": () => grantItems("starter"),         // your game's inventory
};
const app = express();

app.post("/webhooks/coinsflow", express.raw({ type: "application/json" }), async (req, res) => {
  if (!Buffer.isBuffer(req.body) || !verified(req.get("CoinsFlow-Signature"), req.body)) return res.sendStatus(400);
  const event = JSON.parse(req.body.toString("utf8"));
  if (event.type !== "invoice.paid") return res.sendStatus(200);
  if (event.test && !process.env.TESTING) return res.sendStatus(200); // test events only while you test
  const { id, orderId, metadata } = event.data;
  const deliver = PRODUCTS[metadata?.sku];
  if (!deliver) return res.sendStatus(200); // not one of the store's invoices
  try {
    if (!(await delivered(id))) { // your database: one delivery per invoice id, however often this is told
      const item = await deliver();
      await handOver(metadata.customer, orderId, item); // an email, a download page, the customer's account
      await markDelivered(id, item);
    }
    res.sendStatus(200);
  } catch {
    res.sendStatus(500); // CoinsFlow sends it again later
  }
});

app.listen(3000);

verified() is the signature check from the docs, over the raw body. Deliver by the invoice id, never by the order id alone: the same event can arrive again, and after a chain reorganisation an invoice can become paid a second time. Your endpoint must be an https:// address on the public internet; a store that cannot take requests can ask GET /v1/invoices/{id} instead. For an expensive item, wait for invoice.settled, when the network can no longer take the payment back.

Second payments and late ones

Every address we issue is watched for good. A buyer who pays a second time to a paid invoice, or pays after it expired, is not lost: the money is credited to you, and invoice.payment says so. A payment that is short by more than 0.5% makes the invoice underpaid, and the buyer can top it up to the same address; what comes after the invoice expired is credited with paidLate: true. You decide what to deliver in each case; the refund policy says what buyers are told.

What may be sold here

Test it first

Make the invoice with "test": true, then press the buttons on its payment page, or call POST /v1/invoices/{id}/simulate with paid, underpaid, overpaid or expired. Your store gets the same webhooks, marked "test": true, and no money moves. The code above acts on them only while TESTING is set, so a test never hands a product to a real customer.