I just contacted (and got transferred around a total of 5 times in the process) Amazon support to get all the details about what is and is not eligible for the promotion.
Digital games or game items (PSN cards, XIV timecards, etc) ARE NOT eligible, since according to Amazon these are not actually sold and "shipped" by Amazon.
Physical games ARE eligible as these are actually sold and shipped by Amazon.
Physical pre-orders SHOULD be eligible, for the same reason listed above for regular physical games. We'll probably have to see if this actually works, but I did notice a lot of pre-order only games have the promotion on their page.
Hopefully this helps people in choosing what to buy. I will personally be pre-ordering World of FF and and also ordering a regular physical 3DS game or something.