I live in Canada and have purchased dozens and dozens of parts from the US. Never a problem, just make sure you use USPOST. Not UPS, or any of the others. They add brokerage fees that kills a deal.
Ah, just noticed this. I'm in Europe so I often buy stuff in the US to ship over. USPS is by far the cheapest, also insured. About the only thing that's different from shipping inside the US is that you'll need to declare a customs value on the parcel. You can often agree with the buyer what that should be. It has no consequence for you, but the buyer pays import tax from that value + shipping.