They know the rules. When it comes to rescissions, they have to obey. If you are 1 day late, you're stuck. But if you follow the rules, they know they have to abide. Send it certified, keep your receipt of the certified mailing and track it online for acceptance and print out your receipt as well as get the receipt returned via slow mail. Save these items, as sometimes you need to remind them in few months, if you don't get your deposit back. Or if you put on cc, you just call cc and get charges reversed and you have proof of rescission.
If you send certified in time, don't worry.