Guides: barcodes and QR codes in practice
How to get an EAN number, what size a code on a package has to be, what the QR code on an invoice contains. Specific, with sources and the date they were read.
How to get an EAN code in Poland
GS1 Polska issues the number; the code you can draw yourself. What to do, what it costs, and why second-hand numbers can be expensive.
The QR code on a KSeF invoice — what is in it and what it proves
What the QR code on a Polish e-invoice actually contains, how the first code differs from the second, and how to spot one that leads somewhere it should not.