As a buyer, I have had problems getting what I want from US sellers.for the reasons have been mentioned. So what do you do? Don't use UPS, Use regular envelopes for small things like a few Fat quarters, (cardboard flat rate adds unneeded cost), Canadian Customs are a big problem ( divide large purchases, sure it will cost more in postage but will not cause a red flag to customs who will charge you an arm and leg).
I am fortunate that I have a family member that has a Postal Box just over the border and I have had sent item there from sellers that will not ship to Canada. But there are sellers that will not send to a box which frustrates me. Why will US sellers not sent to a postal box. I pay by Paypal as postal or bank orders are a pain for the Sellers as someone has noted.