The reason why US sellers prefer to use UPS is because UPS will come to their door and pick it up. The seller doesn't have to do anything except package it up.
US Postal will come to your door to pick up packages as well depending on the service selected. Print the shipping tag online, tape to box, request carrier pickup. They will come to your house. Pretty simple.
If the buyer requested US Postal and the seller sent it some other manner, IT'S ON THE SELLER.
UPS and FedEx both have large brokerage charges to Canada. I will never ship anything to Canada using those two after the first screw job my customers got.