I have never had a problem shipping to Canada or anywhere. I don't post it in my ads that I will ship here or there, because I will ship anywhere.....hmmm....must have Dr Seuss on the brain!!! LOL
I figure when they say they are from Canada, I tell them the shipping to Canada, they know before they ask that shipping will be more.
I do special things for them occasionally, like sending the item for the cost and not charging shipping. Or only charging shipping that would be Stateside price. It's not their fault that shipping is so much more.
The customs labels are usually just the green ones, they are smaller and easier than the A forms.