I ran into this not long ago when I bought an item from the USA and paid taxes on it when it entered Canada. The item was damaged in transit to me and had to be sent for repair in the USA. If you send an out-of-warranty item to the USA from Canada you will pay taxes yet again when it's returned to you. You pay taxes on the "value added" to the unit... i.e. the invoiced amount for the repair. There is no amount of paperwork you can submit that will save you paying the taxes. This is not speculation, I called Canada Customs and they explained it to me.
The only way around paying is to have the manufacturer bill you for repairs but write up the paperwork as a $0 warranty repair. Many manufacturers will do this for you, no skin off their nose. It is ludicrous that the governement feels compelled to tax the same item over and over again. Finally, do not use UPS or Fed EX Ground, the money you save by not shipping by air will be charged to you as brokerage fees (the brokerage fees are gratis when you ship air).
Best, Jeff
p.s. check out
http://www.canuckaudiomart.com/ there's a decent selection of gear and it's already in Canada