I get USPS stuff from Prime too, but usually on things like kids books. I've never had XPO. We have a bit of an amazon addiction and have 75 orders this year. Before they started doing the USPS last mile or whatever, I would get Premier Courier once in a blue moon, but that hasn't happened since 2012.
Prime you also get a "guaranteed delivery date" and they will compensate you for late orders. I don't think non-prime customers get that protection