Our biggest issue with USPS is theft. We have documneted at leasr five packages that either disappeared entirely at the recieving post office or had the status changed while they looted the box. The post master does nothing.
In my experience USPS employees will rob you blind with no recourse. That said we only ship UPS.
I might send a check via regular mail lol.
Talk to anyone who has worked in a UPS distro facility, and you'll find they're not much different regarding theft, and regarding the care with which they treat your packages.
