I bought a lot of stuff from the US when I was trying to build an electronic AL in the livingroom (complete and utter failure on that count by the way, artificial life needs interleaved and cross-woven micro-networks in a way that I found impossible to replicate on then current serial electronic parts).
The USPS is very good when it's good and decidedly slow when it's slow. I found the lost delivery rate to be very close to that of Australia Post, about 2 or 3%, of course that may have been entirely at Auspost's end. I also noticed that some items took an inordinately long time to arrive. One time I bought an item from New Mexico and paid for fast delivery. I then bought a second item from the seller but paid the normal rate. The normal came after three weeks (that's pretty good from the US which is 12,000km away at it shortest - mainland to mainland). The quick one took eight weeks, just under two months, which is what it would have taken from England in the days of sailing ships.
Every delivery service has exceptions so that should not be taken as normal. There is one thing I do think that the USPS should be criticised for, the very complicated way it calculates postage for parcels. It's easier to get out of Guantanamo than it is to get a price out of the USPS.
Those quibbles aside I've always found the USPS to be a good service that fills me with confidence but Saturday deliveries really are a bit 19th century so I don't know what the problem is with dropping them.