There is a program called Stylizer (visual CSS editing, highly recommended) which inspired this. Their implementation required you to copy to the clipboard (via a Javascript onclick event on the last page of the purchase flow), but the software sniffed the key right out of the clipboard and upgraded itself automatically. This avoids pasting problems. (Useful factoid: many computer users cannot copy/paste, and many who can can only do it with the MS Word copy/paste buttons onscreen.)

I thought "Dang, that is great", implemented it, and then started looking for a way to extend it without requiring people to read the instructions for copying.