You have an accounting package, a CRM and a webshop. All three full of the same customer data, and still they do not talk to each other. So you retype orders, paste invoices into a spreadsheet and keep running behind the facts. An API integration fixes that: it connects two systems so they exchange data automatically. In this guide we explain what an API integration actually is, what it costs to have one built, how it works, and when no-code or custom is the better call. No sales pitch, just an honest picture for small and mid-sized businesses.
Want the wider context first? Read our guide to AI automation for SMEs. And to ask the right questions up front, our checklist for automating a process is a good place to start.
What is an API integration?
An API integration is a connection that lets two software systems exchange data automatically through an API (application programming interface). The API is a system’s digital socket: a fixed set of agreements that lets other software request or write data. Without an integration, a human does that retyping. With one, it happens on its own.
That retyping is not a fringe problem. According to MuleSoft’s Connectivity Benchmark Report (2024), large organisations use nearly 1,000 separate applications on average, of which only about 29 percent are actually connected to one another (MuleSoft). The rest are islands. And every island means manual work, double entry and mistakes you only spot once it is too late.
You have an integration built the moment that manual work starts to hurt, or when a standard integration between your two packages simply does not exist.
What does an API integration cost?
Having an API integration built usually costs between 500 and 7,500 euros, depending on complexity. A simple link between two systems with a clean API sits at the low end. Once you add more systems, more data or strict security requirements, it climbs. Here is a rough guide per type.
| Integration type | Example | Rough one-off cost |
|---|---|---|
| No-code (Zapier/Make) | Webshop posts a new order to Slack | € 0-500 setup + subscription |
| Standard integration | Existing plug-in between two packages | € 250-1,500 |
| Custom, one direction | CRM pulls data from one system | € 1,500-4,000 |
| Custom, two-way + sync | Real-time sync between ERP and webshop | € 4,000-7,500+ |
On top of the build, budget for upkeep. An API changes, a token expires, a system gets an update: then the integration has to move with it. Those recurring costs are what people underestimate most. How to weigh them against the build price is covered in our breakdown of custom software costs. And whether an integration pays for itself, you can work out with our guide to the ROI of automation.
How does an API integration work? (REST, SOAP and GraphQL)
An API integration works by sending structured requests between two systems over the internet. One asks “give me all of today’s orders”, the other answers with that data in a fixed format, usually JSON. It happens fast, on a fixed schedule or the instant something changes.
APIs have become the backbone of the web. Research by Akamai found that the vast majority of all web traffic, around 83 percent, consists of API calls (Akamai). Almost every modern service talks over APIs. So the question is not whether your systems have an API, but how good it is.
There are three common styles. REST is by far the most popular: Postman’s State of the API report (2023) found that more than 85 percent of developers work with REST APIs (Postman).
| Style | Where you see it | Trait |
|---|---|---|
| REST | Most modern apps and web services | Simple, light, works with JSON |
| SOAP | Banks, ERP, older enterprise software | Strict, heavy, lots of structure |
| GraphQL | Modern apps with complex data needs | Asks for exactly what it needs |
Which style your integration uses is dictated by the system you connect to, not by preference. You plug into what the other side offers. That is why a good developer reads both sides’ documentation before writing a single line of code.
Can you build an API integration yourself?
Yes, for simple integrations you can do it yourself with no-code tools like Zapier or Make. They connect popular apps in a few clicks, no programming. The moment it involves custom logic, sensitive data or real-time synchronisation, you hit the limits fast, and then custom is the smarter route.
No-code is enough for plenty of small businesses. In a large share of simple cases, you connect two apps just fine with an off-the-shelf integration. Want to know which tool fits, compare Zapier, Make and n8n side by side. And if you do go for custom code, our piece on no-code versus custom automation shows you exactly where that line sits.
Move to custom the moment one of these is true:
- You handle personal data or other sensitive information you do not want running through a third party.
- You need two-way synchronisation, where a change on either side has to come through cleanly.
- You have custom business logic no standard integration can cover (discount rules, approvals, exceptions).
- Reliability matters: if the integration stalls, part of your business stalls with it.
How secure is an API integration?
A well-built API integration is secure, but security does not come for free. Data travels over the internet, so you handle authentication (who may do what), encryption over HTTPS and access to only the data that is truly needed. A sloppy integration that sends customer data unprotected is a leak waiting to happen.
In practice, a secure integration rests on a few building blocks: a modern login method like OAuth 2.0 or well-managed API keys, encrypted traffic, a cap on the number of calls (rate limiting), and logging so you can see afterward what happened. If you process personal data, the GDPR applies in full: you must be able to show that data is exchanged securely and shared no more than necessary. At TopDevs we build integrations with token management, encryption and logging as standard, not as an upsell.
Which systems can you connect?
In principle any system with an API, and these days that is most of them. Think accounting packages like Exact Online and Moneybird, CRM systems, webshops like Shopify and WooCommerce, ERP packages and marketing tools. Most well-known packages offer an open API. The craft is not in the existence of the integration, but in tying it together so it stays reliable.
Many integration questions are about the books: orders that land automatically as an invoice in Exact Online or Moneybird, for example. If you run a bigger ERP like AFAS, the question is often whether you build custom on top of the standard package and connect it through the API. We cover that trade-off in off-the-shelf versus custom software. A customer portal, too, almost always runs on integrations quietly doing their work in the background.
What to watch when having an API integration built
Before you have an API integration built, check a few things. They decide whether your integration lasts for years or breaks after the first update.
- Does the system have a documented API? Without documentation it becomes guesswork, and guesswork is expensive.
- One direction or two? Pulling data is simple. Keeping both sides in sync is a good deal more complex, and therefore pricier.
- What happens when it goes wrong? Ask explicitly about error handling, automatic retries and logging. An integration fails eventually; what matters is that you notice.
- Who owns the code? Make sure you keep the integration in your own hands. An integration whose code you do not own is one you cannot take with you. Why that matters so much is covered in code ownership and vendor lock-in.
- Who maintains it? An API changes, so agree on upkeep up front and avoid nasty surprises.
And if an existing integration has to be replaced, say because a vendor is retiring its old API? Then budget for costs similar to building a new one, plus the time to untangle the old setup. Starting early spares you a stretch where your process grinds to a halt.
Do it yourself or have it built?
A simple link between two apps you set up fine yourself with no-code. For anything where money, customer data or reliability is on the line, you want custom work with proper error handling and maintenance. That is exactly the difference between a demo that works once and an integration you run on for years.
At TopDevs we build API integrations at a fixed price agreed up front, with code that stays yours. No subscription to your own integration, no lock-in.
TopDevs is a Dutch software agency that builds custom software, AI integrations and automations. Want to spar about which integration fits your systems? Book a free call and we will look together at where the gains are.
