Have "A" and "W". Will trade for "S" or "X".
By having more codes, people don't necessarily need to be stuck with lots of diaper boxes. They can get all the savings in the least possible amount of boxes and then hopefully use the 30% for future orders (if its worth it for them).
Note that lots of people usually have better deals than this one and the only reason they're buying at Amazon with this offer is because it only comes out cheaper for them this way.