You already have a checkout, a subscription biller, or a payouts job that works. Adding a crypto rail to it stopped being a payments decision the moment you picked a provider. What is left is an integration decision, and it turns on one question: what does a crypto payment gateway API actually expose, and how much of the flow do you still have to write?

EukaPay answers that with a REST API organized around invoices, subscriptions, crypto payouts, balance transfer, and customers. This guide walks that surface the way you would scope it in a ticket. It covers the endpoints, the request and response shapes, the webhook events, and the parts your own system keeps owning.

The surface is smaller than you might expect. There is a pay-in side, a payout side, and an event stream that connects both to your own records. Everything else is your application.

A pay-in starts with an invoice.

on

creates one and returns a payment page URL. You send the customer there to pay from their own wallet, and setting

returns them to your order-confirmation page afterwards.

Those are the fields that matter for wiring an existing checkout, not the whole body.

selects the fiat currency the invoice is denominated in.

decides whether you take that invoice as fiat or as crypto, and it is set per invoice, so one account can do both.

carries your own key/value pairs and is never shown to the customer. Your order ID belongs there.

The payout side is

. It is sent as

, not JSON, so plan for that in your HTTP client. Two fields are required:

, one of USDC, USDT, ETH, or BTC, and

, one of Ethereum, Tron, or Bitcoin. You then give either

in fiat or

in crypto, never both.

The endpoint creates one payout per request.

There is no array of recipients and no batch call, so a run of 200 recipients is 200 requests with your own concurrency and retry policy around them. If you would rather not write that loop, mass payouts are a recipient-list CSV upload in the merchant dashboard instead. You fund the payout balance by transferring from the account's main balance.

Every request carries your secret key in the

header, over HTTPS. Keys are prefixed by mode,

or

, which makes an environment mistake visible in a log instead of expensive in production.

Idempotency is the piece most integrations rebuild after their first timeout. Send

on every POST and PUT, using a V4 UUID. The API stores the response and status code against that key, so a retry returns the original result instead of creating a second invoice. Keys can run to 255 characters and are removed after 24 hours.

Two more things to wire on day one. Every response carries an

header, and those IDs are searchable under Integration Logs in the dashboard, so log them next to your own order ID. Errors return

,

, and a

of

,

, or

. Treat

as a conflict to resolve, and back off exponentially on

. List endpoints use cursor pagination with

plus either

or

, and responses carry

and

.

Webhooks are HTTPS POST requests EukaPay sends to a URL you register under Settings, Integrations, then Webhooks. If a delivery fails, EukaPay retries every 20 minutes for up to 2 hours before dropping the event from the retry queue. Build the handler to acknowledge quickly and do the work asynchronously.

Verify the sender before you trust the body. Take the

header, compute an HMAC SHA-512 hex digest over the raw payload using your secret key, then compare the two.

The underpaid and overpaid events separate a finished integration from a demo. A card authorization is exact or it is declined. A crypto pay-in can be underpaid or overpaid, so your order state machine may need a partial-payment state it did not previously have.

and

complete the payout lifecycle.

EukaPay is a merchant acceptance and payout layer, and conceding its edges early is what keeps your scope from drifting. It is not the chain infrastructure layer, it does not create the stablecoins moving across it, and it does not hold your company's operating funds for you.

Three boundaries follow, and each one is a line in your ticket:

Both sides of the API sit on the same platform, which is what keeps this a single integration instead of two. That platform gives you instant crypto-to-fiat conversion at a locked exchange rate to remove your exposure to crypto price swings, support for most major cryptocurrencies like BTC, ETH, LTC, SOL, USDC, USDT on the pay-in side, and settlement in USD, EUR, GBP, and CAD to your bank account. A crypto payment confirms on-chain, so protection against chargebacks is a property of the payment method. Payouts are sent in USDC, USDT, ETH, or BTC, the rate is locked by default on that side too, and they are not tied to banking hours or the banking calendar.

For the endpoint-by-endpoint reference, the

API documentation

is where a developer (or coding agent) should start. For the shorter route into the same platform, the

crypto payment API

walkthrough covers accepting a first payment, and the

stablecoin payments platform

guide covers what to require before you commit to a provider.

Read the

API reference

, then build against test-mode keys while your account completes onboarding and verification.

Create a EukaPay account

, provide your legal business information, generate an API key on the API page, and point a webhook endpoint at your own handler before you touch live keys.

What is a crypto payment gateway API?

It is the programmatic interface a provider exposes so your application can request crypto payments and send crypto payouts without building wallet handling, chain monitoring, or conversion yourself. EukaPay's is a REST API covering invoices, subscriptions, crypto payouts, balance transfer, and customers.

How do I authenticate requests to a cryptocurrency payment gateway API?

EukaPay uses API keys. Pass your secret key in the

header on every request, over HTTPS. Keys are prefixed

or

depending on the environment mode.

Can I create more than one crypto payout in a single API request?

No. The crypto payout endpoint creates one payout per request, so a multi-recipient run means one request per recipient. For a large recipient list, upload a CSV in the merchant dashboard instead.

Can I take fiat on some invoices and crypto on others?

Yes.

is set per invoice and accepts

or

, so one account can do both without a second integration.

What happens if a customer underpays a crypto invoice?

EukaPay sends a

webhook carrying

and

, so your handler can hold fulfillment and decide what to do.

What happens if a customer overpays?

EukaPay sends a

webhook with the same amount fields, so you can release the order and record the difference for a refund decision.

How does EukaPay retry a failed webhook delivery?

If a delivery fails, EukaPay retries every 20 minutes for up to 2 hours, then removes the event from the retry queue.

Is there a test environment for a crypto payment gateway API integration?

Yes. Test-mode keys are prefixed

, and a staging environment is available for development.