We had a customer refuse to pay the duties/sales tax/brokerage on a UPS shipment that was set to "bill to receiver" and UPS charged our account back for this. Not a nice surprise.
We had another customer in Canada not accept the shipment at all, and UPS returned it to us. We got charged duties/tax/brokerage going in to Canada, and brokerage plus return freight coming back. $180 in charges on a $149 sale. Neat.
If possible we ship USPS to Canada, since we have no account with them to charge back to. If we ship UPS we prepay all the duties and taxes and bill those to the customer. UPS Worldship has tools for this, although I don't find them easy to use.
Mostly we try to convince Canadian customers to get it delivered somewhere on the US side of the border and cross it themselves.