You could persist it in the postbox. It would show as “pending” until you get online again.

A lot more difficult! But I think I recall SMS was used to send payments in Africa. But if we assume they’re both connected that makes it easier if that is indeed the case.

Have you seen #[5]​ ‘s PoS? The merchant can take payments on a very simple (cheap) device with no internet connection. The person paying (who has internet) shows the merchant a PIN that the merchant can confirm means payment was made. Very nifty.