Need advice from Canadians selling parts

I have shipped LOTS to US, Europe and Oz. Always Canada post. Shipping within Canada for larger stuff I have used Greyhound. I've never heard of anyone buying from me having to fill out any forms at all before receiving parts.
The time has come, I have been told, to reduce my inventory of spare parts. How do Canadians, ship parts to the USA? I have never sold anything, online.