I know I should be going to actual UPS support for this, but looking at their support page, I'm not even sure where to go, since I don't see any phone numbers, and the chat-bot is useless.
I live in Canada. I sold an Ebay item to an American buyer, and purchased a UPS shipping label through Ebay. Everything seemed normal (I've sold a lot of items on Ebay, but it tends to be a less than once a month thing, and my buyers are usually Canadians), and Ebay didn't inform me of the requirement to include three copies (?) of a commerical invoice, or any other forms, or mention anything regarding duty when I was purchasing the shipping label.
I saw on this page that you can digitally upload customs forms through a UPS account, but since I didn't purchase the label through a UPS account, I assume I would not be able to do that. By the way, the package has no estimated delivery date on UPS's tracking page, and it's past the last day of the delivery estimate for Ebay. The package has gotten a warehouse scan in Lachine every day, for the past 3 weeks.
I did contact Ebay customer service, and the agent didn't seem to think that I had done anything wrong, and that there shouldn't be any repercussions, but that I should try contacting UPS customer service.