I never had a letter saying it was coming back, it just showed up in the mail.
I express mailed my application the first time and it only took 2 days to get to them.... ($16 to mail it)
Being outside canada, something like UPS or DHL would probably be faster. Also, I have seen many conversations that have said they "date it" by the day it was postmarked. so even if it showed up after the "30 days," it shouldn't hinder the application.
Also I always have to wonder if it is "30 business days" or 30 consecutive days. Because when I had put in my extension it said a number of days, and it was correct.... But only if counting business days.