I finally redeemed two iTunes gift cards last night, finding the process generally straightforward. That said, many Macs now have iSight cameras built-in. As proven by Delicious Library, an iSight can also function as a barcode scanner, moving the bulk of the input burden from the user to the computer.
A second barcode (which creates a bit of confusion) would not have to be added if the scratch-off alphanumeric code could be recognized as such using OCR. With foreknowledge of the typeface and the processing power of modern Macs, this seems technically feasible.
In case the camera is in use by another application, the Scan via iSight button is disabled and the application name is displayed.
There would have to be some conditional rules to hide the mention of optical scanning and the attendant button on Macs without a compatible camera.