Availability of USPS is very relative. I'm not the person to whom you addressed the question, but I used to live in a very remote location, and the post office was quite a ways away. In fact, I now live in a place that does have a USPS location, but that location is sublet to an independent business that operates the postal service on behalf of the USPS, and it's not possible to get money orders there.

Depending on the kind of cards you're trying to MS on, different things are available. For example, Citi cards can take debit card payments over the phone.