System Integration for Your Business: How to Connect Your App to Accounting, ERP and E-commerce

Your accounting lives in one program, your sales happen on a marketplace and in your own store, and then there are couriers and online payments. Each system works, but none of them talks to the others - so someone in your company retypes orders, invoices and stock levels by hand every single day. System integration in your business is how you make that data flow on its own, with no human in the middle. In this post we explain in plain language how a new application can connect to what you already run, what an API and a webhook are, what to do when a system has no API, and why a "simple integration" is rarely simple.

What do we usually connect apps to?

In small and mid-sized companies the same set keeps coming up: accounting software (in Poland that means tools like Comarch Optima, Subiekt or wFirma), sales channels (Allegro, BaseLinker, WooCommerce), payments (Przelewy24, Stripe) and couriers (InPost, DPD, DHL). A new application - a CRM, an order panel, a production system - does not have to replace any of it. Its job is to connect: pull an order from the store, issue a document in accounting, book a courier and update stock. That is how the stores we build as part of our e-commerce solutions work, and integration itself is the natural next step after building your own system instead of Excel.

API and webhooks - how systems talk

An API is a system's service window: an agreed way for another program to ask for data or hand it over. "Give me June's invoices", "create order number 1024" - that is what typical API conversations look like. A webhook works the other way around: the system itself speaks up when something happens. A customer just paid for an order? The store instantly pings your application, which issues the document and books the courier. Without webhooks your app would have to keep asking "anything new yet?", which is slower and puts needless load on both systems.

What if a system has no API?

Some older accounting and warehouse programs have no API, or only a very limited one. That leaves two routes. First: file exchange - the system exports data to CSV or XML and your application picks the files up on a schedule (and the other way around). Less elegant than an API and the data arrives with a delay, but it is predictable and cheap to maintain. The second route, a last resort: RPA, a robot that clicks through the program exactly like a human would. It works until the vendor changes the screen layout in an update - then the integration falls apart. That is why we treat RPA as a fallback, never as the foundation of your business.

ERP integration and accounting: the usual traps

This is where ERP integration surprises even experienced teams. What to watch for when planning:

• API rate limits - many systems only allow a set number of requests per minute or hour. Syncing thousands of products needs to be designed around that, or the integration ends up stuck in a queue.
• License fees for API access - some ERP vendors charge extra for the integration module itself or for each additional seat. Check this before pricing the project, not after.
• Two-way synchronization - when data can change on both sides, you must decide which system is the source of truth. Otherwise you get conflicts, duplicates and stock mismatches.
• Responsibility for outages - the integration can fail because any of the connected systems failed. A good integration has monitoring, alerts and automatic retries, and the contract says clearly who reacts and how fast.

What integration costs and why "simple" rarely is

Clients often say: "It is just sending the order to accounting". In practice the edge cases pile up fast: what if the customer enters a wrong tax ID? What about an invoice correction or a return? What if the other side's API goes silent at 3 a.m.? Handling those situations is usually more work than the happy path itself - and it decides whether the integration is reliable or only works in a demo. As a rough guide: a single one-way integration usually costs a few thousand zlotys, an application with a standard set of integrations starts from PLN 10,000, and custom builds with ERP integration and two-way sync start from PLN 25,000. That money comes back as the hours of manual retyping disappear - we covered the math in our post on process automation in a small business. And you can read how we build systems with integrations on our software development service page.

Where to start with system integration in your business?

Not with technology - with the pain. List the systems you use and point to the one place where manual retyping costs the most time and mistakes; for most companies that is orders or invoices. That is where we start the analysis, because that is where integration pays for itself fastest, and further connections are added in stages. Want to know what system integration would cost in your company? Try our quote calculator - you get a ballpark price range on the spot, a first reply from a real person within 24 hours, and a binding quote after a short call about your systems.