The only thing that is a hassle to me is when in line at the PO and you're shipping to Canada, you have to get out of line to fill out the customs forms. Then get back in line to finalize the mailing. That part is just a big hassle especially if you use a busy post office. If USPS could do a better job of providing info, it would help greatly. Mainly it's just a time factor when you're trying to mail out several things.
I don't mind doing it at all but this could be one of the issues. :)