KSeF integration via the API is needed when invoices are created in your own system, online shop or admin panel, when you handle a high volume of orders, or when your sales process is unusual. The deadlines for KSeF, Poland's National e-Invoicing System, are firm: from 1 February 2026, companies with sales above PLN 200 million in 2024 must issue invoices in KSeF, and all companies must receive them; from 1 April 2026 the remaining VAT payers must issue them; and from 1 January 2027 the obligation also covers the smallest companies with sales of up to PLN 10,000 per month. There are no penalties until the end of 2026 — from 1 January 2027, failing to issue an invoice in KSeF carries a penalty of up to 100% of the VAT amount on the invoice, and up to 18.7% of the gross amount for invoices without VAT.
If you invoice in an invoicing programme or in an ERP with built-in KSeF support, you probably do not need to build anything. If the invoice is created in your own system, it has to be connected to KSeF via the API.
When does KSeF become mandatory and when do penalties start?
The timetable is spread over two years, but the first date applies to every company: from 1 February 2026, invoices in KSeF must be received, regardless of sales volume.
| Date | Who it applies to |
|---|---|
| 1 February 2026 | Mandatory issuing of invoices in KSeF for companies with sales above PLN 200 million in 2024 |
| 1 February 2026 | All companies receive invoices in KSeF |
| 1 April 2026 | The remaining VAT payers issue invoices in KSeF |
| 1 January 2027 | The obligation covers the smallest companies with sales of up to PLN 10,000 per month |
| 1 January 2027 | End of the penalty-free period: up to 100% of the VAT amount on the invoice, and up to 18.7% of the gross amount for invoices without VAT |
For most companies the obligation is already in force, and penalties are waived only until the end of 2026. These are the last months to get the integration right without the risk of penalties. Confirm with your accountants which dates apply to your company specifically and to what level of sales — CodePort handles the technical side and does not advise on tax matters.
What is KSeF integration and do you really need it?
KSeF integration means connecting your system to KSeF, Poland's National e-Invoicing System, via the API: the sales data you already hold in your database is turned into an invoice in the required structure, sent to KSeF and tagged with the KSeF number returned. Without it, someone has to retype the same data into another programme by hand.
Before you order anything, answer one question: where exactly is the invoice created?
| Where the invoice is created | What is enough |
|---|---|
| An invoicing programme with KSeF support | The vendor's ready-made module — no development needed |
| An ERP with built-in KSeF support | A ready-made module in the ERP |
| Your own system, panel or online shop | Integration via the API |
| An unusual sales process or a high volume of orders | Integration via the API |
If you fall into the first two rows, do not buy an integration just in case. If you fall into the bottom two, it is worth closing this off before 1 January 2027, when penalties come into force.
Which situations in practice require integration via the API?
Your own system or panel
You have a web application in which you handle orders, contracts or subscriptions, and it is that application that generates invoices (even if only as PDFs). The data is already there; all that is missing is the layer that turns it into a document in the KSeF structure and sends it on. This is often the case in companies that once built themselves a bespoke web application or panel.
An online shop with a high volume of orders
With a high volume of orders, issuing invoices manually stops being an option — not because of the regulations, but because of the hours it takes. Integration moves that to the system, and people are left with the exceptions: credit notes, returns and unusual orders.
ERP and accounting software
ERP integration with KSeF can be needed even when the vendor offers a module — for example when part of your sales goes through a channel the ERP does not support, or when the invoice data is created in another system and has to reach the ERP first. In that case the real task is connecting two systems to each other, not KSeF itself.
Selling on Allegro
The API of Allegro, Poland's largest online marketplace, allows up to 10 PDF invoices to be attached to each order, each up to 3 MB in size. As a result, an invoice generated in your system reaches the buyer automatically, without uploading documents one by one. You will find more about connecting a shop, an ERP and payments on the page about system and API integrations.
What does KSeF integration cover, step by step?
The scope we deliver at CodePort looks like this:
- Generating invoices in the FA(3) structure from the data already held in your system — no retyping and no second place for the same information.
- Sending to KSeF and saving the KSeF number against the document, so that every document in your database carries confirmation that it went through.
- Error handling and retries, so that no invoice gets stuck unnoticed. This is the most underrated element — until the first silent send failure happens.
- Downloading purchase invoices into the system, because from 1 February 2026 invoices also have to be received.
- Offline mode handling in case KSeF is unavailable — sales cannot stop when the system on the other side does not respond.
That is the minimum list. The rest depends on what happens at your end with credit notes, prepayments and returns — and on how your accountants describe those processes.
How much does KSeF integration cost?
We calculate the cost simply: number of hours × €46 net for development. The number of hours depends on where invoices are created, how many there are and whether your system has an API.
Euro amounts converted at the NBP mid-market rate of 14 September 2026 (€1 = 4.3391 zł) and rounded. We invoice in PLN.
For example: 40 hours of work is €1,850 net. This is an indicative calculation, not an offer — we give specific hour ranges after reviewing your systems.
What genuinely pushes the cost up:
| Factor | Why it increases the number of hours |
|---|---|
| No API documentation | The system's behaviour has to be worked out by trial and error |
| An old system with no API | Another route is needed: file import/export, a direct database connection or building an API on top |
| Large data volumes | Queueing, performance and resilience to outages |
| Many exceptions in the process | Every exception is a separate path to build and test |
If your system has no API, that is not the end of the story — there are other routes: file import and export, a direct database connection, or a new API built on top of the existing system.
And maintenance?
Vendors change their APIs, so an integration is not a "do it once and forget it" project. Typically it takes 5–15 hours a month for monitoring, updates and fixes. For example: 5–15 hours × €46/h net is €230–690 net per month — the actual figure depends on the number of connected systems.
If you also want to take other repetitive document-related tasks off people's hands, see how business process automation works — invoices are usually just one of several processes that eat up hours.
What does the integration rollout look like and how long does an estimate take?
- A conversation and a review of your systems. We check the API documentation, the data flow and where documents are created today. You get a price range and a schedule within 2–3 working days, and a reply to your initial enquiry within one working day.
- A data flow diagram before any code. We agree on paper where data flows from and to, and what happens when one of the systems does not respond. This stage catches the most misunderstandings.
- Sprints. Every 1–2 weeks you see a working part of the integration, not a progress report.
- Launch. Going live in production, error handling and monitoring.
We invoice in stages, work under a contract and an NDA, and full intellectual property rights to the code pass to you once payment is settled — the code goes into your repository. With an integration to a tax system this matters in practice: you are not held hostage by a single supplier.
What to prepare before talking to a developer
The more of this you have, the narrower the price range will be:
- The names of the systems in which invoices and orders are created (your own system, shop, ERP, accounting software).
- Whether those systems have an API and whether you have the documentation and access to them.
- The approximate number of invoices per month and your sales channels (shop, Allegro, direct sales).
- A list of exceptions: credit notes, prepayments, returns, foreign sales, consolidated invoices.
- What you have agreed with your accountants: exactly when your company has to start issuing invoices in KSeF and who checks that the data on the document is correct.
The last two items are a job for your accountants or tax adviser. We translate their decisions into technology. The quickest way to gather it all in one place is to fill in the project brief — on that basis we will prepare a price range and an initial specification.
What not to do
Do not wait until December 2026. Penalties start on 1 January 2027, and an integration is not just code — it is also testing, fixes and the first month in production, when exceptions surface that no documentation describes.
It is also not worth building the integration "along the way" during a major system rewrite. KSeF can usually be connected to what you already have, without replacing tools that work.
Describe where your invoices are created and which systems you work with — you will get a price range and a schedule for KSeF integration within 2–3 working days.
Frequently asked questions
KSeF integration — when is it really needed?
Receiving invoices in KSeF is mandatory for all companies from 1 February 2026, issuing them is mandatory for the remaining VAT payers from 1 April 2026, and for the smallest companies with sales of up to PLN 10,000 per month from 1 January 2027. The date that applies to your company should be confirmed by your accountants.
How much does KSeF integration cost?
The cost is the number of hours × €46 net for development; for example, 40 hours of work is €1,850 net. We give an exact hour range only after reviewing your systems and API documentation, because a system without an API requires a different and usually more labour-intensive route.
Does KSeF integration with accounting software make sense if it already has its own module?
If invoices are created directly in that programme, the ready-made module is enough and there is nothing to develop. Integration is useful when the invoice data is created earlier in another system and someone retypes it by hand today.
What happens if KSeF is unavailable during sales?
The integration includes offline mode handling and send retries: documents wait in a queue and go to KSeF once it is available again, and the system flags those that did not go through so that nobody overlooks them.
What should I do if my system has no API at all?
There are other routes: file import and export, connecting directly to the database, or building a new API on top of the existing system. The choice depends on how the system is built and whether you have technical access to it.