I just had to do the same thing from Atlanta.
I set up an account with Fed Ex then set up the shipping labels to and from Canada at the same time, using the estimated weight of the return package (if it is just papers, .5 lbs is usually adequate). If the weight is off, they'll adjust the cost to you if you have an account. You won't get charged by FedEx for the return until it's been mailed.
Then I went to FedEx, grabbed two FedEx mailing envelopes and asked for assistance since it is International, they added a plastic sleeve to the back of each and inserted the customs info that printed with the labels (you can print these or ask them to when you arrive, they'll access your account to do it).
Then they affix the labels and the return envelope gets folded in half and stuck in the other envelope.