The small flat rate boxes are nice.
Maybe say if it won't fit in a standard mailbox it should be UPS/Fed-Ex
That makes a lot of sense.
ups/fedex use the usps a lot to cut their own costs... doing away with parcel delivery would probably make prices rise significantly across the board.
If it costs more to use UPS/FedEx then so be it. The idea that their costs are kept artificially low because they get to access the resources of a taxpayer supported system is actually offensive. Let the burden of the cost of those delivery services fall on those who use those services.