Payment API Integration

Payment API integration for custom checkout flows, SaaS platforms, mobile apps, and any application that needs to initiate card transactions programmatically. Brookside merchant accounts connect through industry-standard gateways — Authorize.net, NMI, and others with RESTful API support — giving your development team full control over the checkout experience with interchange-plus pricing behind every transaction. Whether you need to integrate payment gateway into website checkout, build a gateway for a SaaS product, or connect a payment solution to a mobile app, the architecture options are the same — what changes is the processing cost behind them. According to Federal Reserve interchange fee data, the processing rate behind your gateway matters as much as the integration itself — most developers connecting through Stripe or Square are paying 0.4–0.8% more than necessary at meaningful volume.

For merchants who have outgrown flat-rate aggregators, Square alternatives built on real merchant accounts typically cut effective rates by 20–35%.

Supported gateway options include Authorize.net, NMI (Network Merchants Inc.), and other processors with RESTful gateway support. These gateways provide tokenization, hosted payment fields, webhook notifications, and server-to-server processing — the building blocks for any custom integration regardless of your application stack. These gateways support recurring billing, stored credentials, and webhook-driven reconciliation.

For developers migrating from Stripe, Authorize.net and NMI use similar concepts — tokenization, webhooks, idempotency keys — with different endpoint structures. Migration typically involves updating credentials and endpoint calls, not rebuilding checkout logic from scratch. The bank merchant services call post explains why your existing banking relationship is rarely the best option for gateway-integrated merchant accounts. When you are ready to switch, read how to switch payment processors without losing a day of sales.

When you connect through a payment facilitator like Stripe, flat-rate pricing applies to every transaction regardless of what your code does. With a dedicated merchant account, interchange-plus pricing means you pay actual card network costs on every transaction — a difference that typically runs 0.4–0.8% at meaningful volume.

The Math for SaaS and E-Commerce

For SaaS platforms and payment API for ecommerce businesses processing significant monthly volume, the cost difference between interchange-plus and flat-rate pricing is often substantial enough to justify the integration work involved in migrating processors. A dedicated merchant account consistently outperforms payment facilitator pricing above $10,000/month. Use the Effective Rate Calculator to estimate the difference at your actual volume. For why flat-rate pricing consistently costs more, see how tiered pricing works. Understanding what a merchant account is and how it differs from payment facilitator platforms is worth reading before you commit to an architecture. See CFPB guidance on card payments for consumer protection context on card-not-present transactions.

Integrations generally fall into two architectural patterns:

• Hosted payment fields — embed payment input into your checkout using iframes or JavaScript served by the gateway. Card data goes directly from the customer’s browser to the gateway without touching your servers — significantly reducing PCI compliance scope. Best for most businesses building their first secure payment API integration.
• Server-to-server integration — sends transaction data directly from your application server to the gateway, offering more control over the checkout experience but requiring stricter PCI compliance measures on your infrastructure. Worth evaluating for businesses with existing PCI infrastructure or specific checkout requirements.

PCI Scope and Tokenization

Tokenization tools reduce your PCI compliance scope by keeping sensitive card data out of your application entirely — replacing the card number with a non-sensitive token your application can store and reference for recurring charges. Both integration levels support this tokenization model. Brookside can help you understand the compliance implications of each approach before you commit to an architecture. If you are unsure whether a dedicated merchant account makes sense at your volume, read do I need a merchant account.

The most common scenarios we support: custom checkout flows built on WooCommerce or headless commerce platforms, mobile apps that need native in-app payment processing, SaaS products that handle billing for their customers, and internal business systems that need to initiate or record payments programmatically. Requirements differ by use case — each has different requirements around hosted fields versus server-to-server processing, webhook handling, and stored credential management for recurring charges. For specific problems developers encounter with Square and Stripe account stability, read what happens when Square freezes your account — the account stability risk that comes with payment facilitator models applies equally to gateway-integrated businesses.

Integration Support and Onboarding

Technical questions during payment API integration are handled by the same team that manages your account — not a generic support queue. If you are migrating from Stripe or another processor, we review your current integration and identify what changes are needed to connect to your new merchant account. Brookside provides integration documentation and technical support during migration to ensure the new connection is processing correctly before the old one is decommissioned.

Frequently Asked Questions

See What Your API Transactions Actually Cost

Send your current statement and we’ll show you what interchange-plus pricing saves over Stripe or Square at your volume — before you commit to any development work. No obligation.

Or call (833) 382-1992 · hello@brooksidepayments.com