Just called them and it is all automated but pretty helpful. Of course it didn't say anything about crossing the US border and coming back

(wishful thinking) but to extend your stay in Canada, simply apply online on the website that you mentioned Leon.
At this point it will take 62 days until you hear back from them. The good thing: you are allowed to stay in Canada until a decision has been made. They basically give you a bridging visa for the period of time your visa has been officially expired.
The only important thing is to apply for extension 30 days before it expires.
Which I am going to do now. Cost is $75 by the way.