What is an API integration?

An API integration earns its keep in the least visible part of a business: retyping. A web shop taking 15 orders a day, where someone re-enters each order in the accounts, spends three minutes per order, 45 minutes a day, more than 16 hours a month. And retyping produces errors: the wrong VAT rate, a missed invoice, a postcode with a digit too many.

An API is a program's service counter. It does not let anyone into the kitchen, but it accepts set questions in a set format from whoever holds the right key. Your shop asks the accounts package "create this invoice", and it answers "done, here is the number". The integration is that conversation, repeated thousands of times without anyone watching. Most web APIs today follow the style Roy Fielding described in his doctoral thesis in 2000: ordinary web addresses, ordinary requests, answers in a readable format. What each route costs to build and run is set out on API integration services.

The connections that pay back first

The second row is turning from nice-to-have into law across Europe. Belgium made structured e-invoicing between VAT-registered businesses mandatory on 1 January 2026, over the Peppol network. Germany has required every business to be able to receive e-invoices since January 2025, with issuing mandatory from 2027 for larger firms and from 2028 for all. France follows from September 2026, and the EU's VAT in the Digital Age package makes e-invoicing compulsory for cross-border trade within the EU from July 2030. If you invoice from a web shop or your own software, each of these means an integration. Belgium's rules are explained in our e-invoicing guide, and the Peppol check shows whether a Belgian customer can already receive.

Integration platform or custom build

For simple flows between well-known packages there are integration platforms. You pick a trigger ("new form submission") and an action ("create a contact") and pay monthly or per run. Zapier starts at $19.99 a month on annual billing for 750 tasks. n8n's hosted Starter plan is €20 a month for 2,500 executions, and if you host it yourself you only pay for the server. Which of the main three fits you is covered in Make, Zapier or n8n.

• Choose a platform ifyou need a handful of flows between packages the platform already knows, with few rules and modest volumes.
• Choose a custom build ifthe flow has rules no platform knows (VAT per country, stock across three warehouses), carries a lot of data, or your own software is the hub.
• Choose a file export ifthe other system has no API: a daily file is slower, but beats retyping.

Add AI and a flow can read emails or process invoices automatically. And if you want the other system to tell you when something happens instead of you asking, that is a webhook.

Integrations fail quietly

An integration is never finished. When either side changes, a new field, a new API version, an expired key, the connection has to follow. The danger is that a broken integration makes no noise: you notice when a customer calls about an invoice that never arrived. Three things prevent that: an alert on every failure, a log of what went through, and a named person who owns it.

Two examples from our own work show the range. For No.4 Lange Ast, a holiday home in Flanders, the website pulls availability from the Airbnb and Booking.com calendars so guests can request dates directly without commission. The transport platform we built for DA Logistics prepares its own bundle for the accountant and sends drivers their messages by email.