[CaptainNegative]

My understanding is that Amazon is not the one printing these cards. Unless they go out of their way to scratch the card off themselves and then cover it back up or create a knockoff, the pack of activation cards they receive are all effectively indistinguishable from Amazon's point of view. They could track which of the various indistinguishable cards was shipped where, but that doesn't help towards determining who was shipped any given code.

The above attack might be a possibility if you're already being actively tracked by the NSA, but at the very least this approach gets you some degree of forward privacy in case the NSA only starts hardcore snooping after the card was already delivered to your door. Whether or not it is a useful degree of privacy is out of my area of expertise.