Hi...Canadian stamps will only work for your letter TO the USA. For the return you will need to have either USA stamps or IRCs. A lot of members on here often sell them or I believe its easy to order them from the USPS site. It's just cheaper to get US stamps than IRCs and your chances of anything increases significantly (in my opinion).

Hope that helped.

I was wondering can you use canadian stamps to send something back from the US or do you need an IRC?