One reason many postal services are interested in this is that they tend to have a very wide physical footprint in a time where banks are increasingly closing b

One reason many postal services are interested in this is that they tend to have a very wide physical footprint in a time where banks are increasingly closing branches. In many countries post offices act as a generalised "service provider" where third parties can contract to have in-person transactions done. Often they are processing various transactions for existing banks anyway. I'm not in the U.S. but yes I expect that Walmart would be a direct competitor in providing these sorts of services.