Portugal has mandatory electronic receipts. By this I don't mean email receipts, I mean that all receipts have a code on them that is then also available to be looked up on the government's side (e-Fatura is the app for this) for tax reasons. I think it's fairly simple though, just registers the total amount, the seller, and how much VAT was paid.
However I assume this can work offline with the data being uploaded later though, as basically all the small supermarkets and shops were still open here (_incredibly_ chaotic though), and on the big supermarkets card payments were working (TBF, even the free wifi was working there, I guess they probably have some satellite connection).
