Banking from the post office is a pretty common thing and makes a lot of sense as it gives people a public, non-profit banking avenue for the poor and under served.
I would agree if there were more post offices, but the USPS has been closing them over the last twenty years to cut costs. They serve wide geographic areas and it is easier to find a local credit union than a post office these days. The poor would be better served with Internet banking.
Many people do not have reliable internet access.
Perhaps a potential USPS credit union would disrupt payday loans.