The complete guide to Payment API

What a payment API actually is

A payment API is the programmable interface your application uses to move money: it turns a customer's card, bank account, or wallet into an authorized, captured, and settled transaction without your code ever handling the raw account details. Instead of building connections to card networks and banks yourself, you send a request and it does the heavy lifting. The Flux payment API exposes cards, ACH, and stablecoins through one consistent interface, so the same integration pattern works across all three rails.

Think of it as the seam between your product and the financial system. Your app knows about orders, customers, and carts. The payment API knows about authorizations, captures, refunds, payouts, and settlement timing. A good one keeps those two worlds cleanly separated, so you can change your checkout without touching your ledger, and the other way around.

The core objects you will work with

Most payment APIs, Flux included, are built around a small set of objects. A charge represents an attempt to move money from a payer to you. A token represents a stored payment method, captured securely so you can charge it again later without holding the card number. A refund reverses a charge in whole or in part. A payout represents money moving out to a recipient. A webhook is an event notification the platform sends your server when something changes.

Once you understand those objects, most integrations look similar. You create a token from a payment method, you create a charge against that token, you listen for webhooks to confirm the final state, and you reconcile against settlement. The details vary, but the shape is consistent, and that consistency is what makes the first integration teach you the second.

How does a payment API move money?

The lifecycle usually has three phases. First, authorization: the network checks that funds or credit are available and places a hold. Second, capture: you tell the platform to actually collect the authorized amount, either immediately or later, for example when you ship. Third, settlement: the money lands in your account on the rail's own clock.

That clock matters for cash flow. With Flux, card transactions settle in 1-2 business days and ACH transfers in 1-3 business days, while stablecoins arrives in the merchant wallet instantly. Because the API abstracts the rails, your code path for taking a payment stays largely the same even though the money behind it moves on very different timelines.

One integration for cards, ACH, and stablecoins

The practical advantage of a unified payment API is that you write your checkout once. Adding ACH for customers who prefer to pay a large invoice straight from their bank account does not mean a second vendor and a second reconciliation process. Accepting stablecoins does not mean bolting on a separate wallet integration. They flow through the same objects and the same webhook stream.

That consolidation also simplifies your books. Flux syncs transactions to QuickBooks, so the payments you take through the API show up in your accounting without a manual export step.

What does a payment API cost?

Pricing on a payment API has two parts: the per-transaction fee and the fixed platform costs around it. Flux charges a flat 2.9% plus 30 cents per transaction, with volume discounts and custom interchange-plus pricing available as you scale. There are no setup fees, no monthly fees, no minimums, and no contracts, so the cost of integrating is your engineering time, not a subscription.

Where local surcharging rules allow, Flux also lets you pass the processing fee to the customer at checkout, which changes the economics for some businesses. That is a policy decision as much as a technical one, so check the rules for where you operate.

Where PCI scope lives

The riskiest part of any payment integration is the card data itself. Flux is SAQ-D Level 2 PCI DSS certified, and it captures card details inside origin-isolated iframes hosted on payments.fluxpayments.com. Because the sensitive fields render on Flux's domain, the raw card number never touches your servers or your domain, which keeps the environment you have to secure small.

For your team, that means the payment API is not just a convenience layer. It is also the boundary that keeps the hardest compliance obligations on the platform side rather than yours.