The post office idea makes a lot of sense to me. There are a lots of people with no bank account but could use a pay app that isn't going to screw them.
The unbanked in the USA don't have a bank account for a variety of reasons. Typically
1. Mistrust and fear of "being in the system"
2. Having a legal judgment against them e.g. child support, fines, overdue rent
3. Avoiding showing income to not lose income-defined government benefits
4. Hiding income from tax authorities
