System Integration: How to Connect Invoicing, E-commerce and ERP

In short: system integration means your company's different programs (invoicing, e-commerce, ERP, CRM) exchange data automatically, so nobody copies it by hand. It usually happens through APIs, and it's where you can save the most time and errors.

In many companies the systems are "islands": the web shop lives separately, invoicing separately, inventory separately. Colleagues play the bridge between them with manual data entry. That's slow, expensive and error-prone.

What is an API?

An API (Application Programming Interface) is a gateway through which one program lets another program request data from it or send data to it. For example, the web shop hands an order to the invoicing tool through the API, which creates an invoice and returns the invoice details.

Typical integrations

• Web shop → invoicing: an order automatically produces an invoice.
• Web shop ↔ inventory: stock updates in real time in both directions.
• CRM → ERP: a closed deal automatically becomes an order.
• ERP → accounting: invoices and entries transfer automatically.
• Supplier / courier systems: shipping and tracking automated.
• E-mail and notifications: automatic messages on status changes.

How a good integration is built

• Agree on the data model. Which system is the "source of truth" for which data?
• Authentication and security. API keys, encrypted connection, minimal permissions.
• Error handling. What happens when the other system is unavailable? Retries, queuing, alerts.
• Logging. Every transfer should be traceable.
• Testing. In a test environment resembling production data.
• Monitoring. If the integration stops, you know immediately.

When is a custom integration worth building?

If two systems have a ready-made connector, it's often enough. You need a custom integration when:

• there is no ready-made connector, or it doesn't cover your process;
• there are complex rules (pricing, exceptions, approvals);
• volume is high and reliability matters;
• you must connect several systems at once.

The most common pitfalls

• Double data entry stays, because nobody clarified which system is the boss.
• No error handling, so a missed sync silently causes data errors.
• The API changes, and nobody maintains the integration.
• No monitoring, so it's down for days before anyone notices.

What does it cost?

A simple integration is a few days of work; a complex one touching several systems can take weeks. A large part of total project cost is often in the integrations, as I wrote in the cost article.

If you'd like to connect your systems, see what I build or book a consultation. It's also worth reading about automating admin work.