NO WORRIES! They are not required to provide you with any written documentation of your rescission request. They are however required to act on your request, and with most Timeshare systems you will receive your refund within a 45 day time frame.
As long as you mailed your rescission letter within the allowed period of time, and have retained a Postmarked copy of your USPS mailing receipt (which it sounds like you have) then you are all set.
Now, relax, become a member of TUG, and go out and celebrate
.