Placing that on a thank you page wouldn't be as effective. You have already payed and laziness is a major factor. I think that the middle of paying process is the best spot to place it. It doesn't need to be so big and intrusive, maybe just a checkbox placed above the "Checkout" button would do the trick and still get a lot of conversions.
I do like the idea - but would rather see it on a thank you page (e.g. after subscribing) or (after checkout) not in middle in case a customer bails.