Buyer pays. It's up to the buyer/importer to know the rules of import for their own country. It's not up to the seller to remind them of that, to find out how much that would be and certainly not pay for it.
Word it nicely but it's their responsibility not yours. Remind them that it's not a hidden or extra cost but a tax imposed by their country that they are responsible for knowing about and paying when they import goods.
How much was the item worth? Do you think it would do any good to offer your experience with Canadian customs and taxes? As in our limit is $20 and while some higher priced items arrive without money owing some don't.
You didn't up the value of the item on the customs form did you? Some US sellers do that thinking they'll benefit from the supposed extra insurance. The value on the form should only be what the customer paid before shipping costs.
