I too hope that your rescission instructions did not clearly specify and require the use of certified U.S. Mail.
I'm not sure why you chose to use FedEx instead, but what's done is done.
It does not matter when your rescission (cancellation) correspondence is received. What matters is
postmark date. What you would ultimately need to prove (if necessary) is the mailing date of your cancellation and I don't know what (if any) hard copy documentation FedEx may have provided to you specifically identifying that date.
If you get any phone calls from Westgate, don't answer them. Such calls will actually be a good sign though, indicating that they acknowledge your rescission attempt and the sales weasels now want to attempt to salvage their sale (don't even talk to them --- and do
not call them for a "status update" at any time).
I hope you resent the cancellation by certified U.S. Mail yesterday (08 January), as LannyPC very wisely suggested above.