You pay him to buy it for you and ship it to you. Pretty simple. Shipping from the US to Canada is anywhere from $10-25 depending on the service used (USPS First Class International is cheap - $5-15 for small items; USPS Priority International costs a bit more - ~$20-30, but comes with a tracking number and is a little bit faster).

Alternatively on another enthusiast forum I know some overseas people use mail-forwarding services, where they get a third-party located in the US to get the item "shipped" to and then ship it out overseas, but I've never heard of a Canadian using one.