There is no media mail outside of the US but you are shipping to Kentucky so using media mail is fine. When it reaches Kentucky the gsp will process it and send it with another carrier to Canada..not usps.
As far as combining shipping, the seller does not have any control for combining shipping with a gsp package. If the buyer puts their items in a cart and pays that way, the international shipping will be combined but the domestic portion will not so the buyer would pay full price for shipping each item to Kentucky.
In a situation like this it would be best for you to cancel the first sale and then to put both items in one listing and put the appropriate shipping in the listing so that the buyer is paying the correct amount.
