I've discovered something frustrating with USPS vs UPS for buying items from the US for shipment to me in Canada.
I'll use a recent example of a keyboard (I forget which, I've bought 3 keyboards from the US in recent times). I don't know the specific names of the shipping services, just that I always pick the "lowest price of the group" since it's always sufficient.
USPS quote was $28 or thereabouts (USD)
UPS quote was $18
What's the obvious choice here? UPS, of course!
Keyboard arrives next day or 2 days later and BAM, $20 Cash on Delivery charge.
Another keyboard, seller and I had arranged to do it by UPS (same 18 dollar figure) and seller goofed...sent it by USPS instead, costing them more than I paid. I offered to make up the difference but seller refused to take it.
It took longer...but no Cash on Delivery charge. Ultimately, USPS despite being priced higher cost less in the end because there was no ridiculous fee on it.
USPS parcels will sometimes get hit by
GST/PST by the Canadian government, sometimes even duty in extreme cases, but at least there's the chance they might not (and for the majority of items it seems they do not).