If I have a US Credit Card and I make a purchase in Canada, I know I won't get the BEST exchange rate. But if I return whatever I purchase and the store initiates the refund process, will I get a full refund? Or will I be out some fees?
It depends on your credit card detail, you'll need to check the agreements.
Typically, credit card charges are converted with the (very good) bank exchange rate, and then a 2.5 - 4% fee is tacked on (without explicitly showing on the statement, so you think you got a poor exchange rate). Some credit cards offer 0% fees, and they are obviously quite useful if you have often international charges.
Check your agreement, and you will see the exact percentage added on for international charges.
Regarding the reimbursement: there are two possibilities, 1) they reverse the original charge, which would mean it simply gets removed from your bill, at zero net cost to you; or 2) they add a new charge with the negative amount, in this case it gets converted again, with a second fee of course, and the fee will not be negative; so you will net be out of two times the fee. Plus the currence risk, as the exchange rate might have changed inbetween, but this could be good or bad for you.
The first option - reversal - is only possible for a limited time (some days), but you could try to talk them into doing this. If they know how to do it, it might save you two fees, but they might well say it's too late or they don't have that option, true or not, you can't force them.
will I get a full refund?
No, why should you? The ship will give you the refund in the currency they originally charged and the credit card company will give you funds in the credit card company for the exchange rate AT THE TIME OF PROCESSING THE REFUND.
Sucks? Yeah, the world is not fair. This is how it is handled and this is how you agreed to have it handled and the credit card company is not involved in you walking into a shop returning goods.