on the subject of QR codes, you could just print a separate code above each item in the vending machine, then all a customer has to do is scan the code and push 'confirm' and the machine will vend whenever it receives a payment on the particular account.
Of course this means if you ever change prices you need to re-print codes with the correct payment value on it, but it does mean that no codes need to be entered into the machine at all.
Of course this means if you ever change prices you need to re-print codes with the correct payment value on it, but it does mean that no codes need to be entered into the machine at all.
I love that idea! Point your camera at the product, confirm payment, and you're done.
RE: changing prices: I wonder if you could make little QR-code-capable LCD or e-ink displays cheap enough to put 20 of them in a vending machine.
It'd also be a great opportunity for practical jokes, like making the machine vend... oh, I dunno, NerdsTM along with their popcorn.