Integrating with Poland's KSeF: five things that aren't in the docs

Poland's KSeF system requires mandatory structured e-invoicing, with a system called KSeF validating invoices and assigning numbers. Engineers should be aware that batch sessions return a reference number, but individual UPO (Urzędowe Poświadczenie Odbioru) documents prove invoices were delivered. Per-invoice status tracking is necessary due to individual validation and potential duplicate invoice numbers. Engineers should also be aware of the BrakID identifier, which does not indicate a consumer invoice. Budget for UPO fetches in queue design.