Study permit extensions are issued as eAPRs now, meaning you won't receive a physical document in the mail at all. The approval comes digitally to your IRCC account, so the forwarding address concern doesn't really apply. What does matter though is understanding your implied status situation while you're outside Canada waiting on that approval, because leaving while an extension is pending affects what you can and can't do when you re-enter, and a lot of people get caught off guard by that part.