And if they do I bet we will see a lot of people "borrowing" receipts from those who do buy at the show so they can show "proof" and get their sets slabbed. because I doubt PCGS will want to keep your receipt after all "I need the receipt for my records." So it will be show the receipt, get the submission in, pass the receipt to someone else.
All PCGS would have to do is to stamp the receipt with their logo once they accept the order, any receipt that has all ready been stamped can not produce any more submissions for the “show labels”. Problem solved.