[FooHentai]

The variety of products shouldn't matter though - You only need to know unique code and number of times redeemed. There is no need to track it back to an individual product.

For example, if you ship the unique codes with your products via a small card in the packaging, or a small product-neutral sticker applied to the outside, the end result is the same.

I hear you on the volume problem but I'm not sure it's a significant issue. A key/value store meets the need and will scale to billions while remaining space and compute efficient.