I was at Canada Post yesterday mailing a return in Canada.
On the door was a sign that any US mailing (except letters) required prepayment of US Tarrifs. Canada Post required the person shipping to produce proof of what the US Tarrif was (either on an Ap on their phone or printed). The clerk told me they've been told to caution the person shipping that any mistake and the package will be returned from the US and the shipper charged for the return shipping. Lastly, apparently there is a glitch and so if you mail any photos from Canada to the US, for Tarrif purposes photos are considered taxable and must be declared.