None of the cost goes to Poland's Ministry of Finance. The ministry says companies may build their own KSeF tools without any agreement with it, and it publishes the API 2.0 documentation on GitHub. You pay for work in your system: turning an order into a valid FA(3) file, getting it to KSeF on time even when KSeF refuses or goes quiet, and storing the returned KSeF number and UPO.
What changes the scope of a KSeF integration
| Factor | Why it weighs in the quote |
|---|---|
| Your data | KSeF rejects files that break the schema but, as the ministry notes, does not check the buyer's details, so validation is built on your side |
| Invoice types | FA(3) has seven types, and advance and settlement invoices and their corrections each need their own linked path |
| Several tax IDs | each NIP is a separate context with its own access and limits |
| Sales peaks | an interactive session takes 10 invoices a second, 30 a minute and 180 an hour, so busy days go out in batches |
| Offline modes | an offline24 invoice must reach KSeF by the next business day, after an announced outage within 7 business days of its end |
Upkeep after launch
On 23 September 2026 three API versions reached production at once: 2.7.1, 2.8.0 and 2.8.1. Unmaintained integrations break the day such a change goes live, so we track the announcements and adjust under a maintenance plan or as a short job. Bugs in our work are covered by a standard 90-day warranty, and the code is yours once the work is accepted and paid for.
For hosting companies, Flaem sends FA(3) invoices to KSeF straight from billing, corrections and advance invoices included.
Our KSeF integration page covers connecting your store, billing system or ERP.
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