Hi,
In the shipping section for International, select (clickoff) Worldwide. Below that do a Flat Rate with selected service for Canada only (clickoff Canada) at $1.60 and do another service Flat Rate to all the others (clickoff Worldwide) for $6.50. When you do Preview, you'll see that Canada has its correct rate as well as a Worldwide (all the rest) with correct rate.
It's exactly how I use to do International Flat Rate with USPS 1st Class, so hopefully it still works the same. Double-check the listing when its on eBay, i.e. select Canada for ship-to and then some other country for rate checks.
Danno