I heard on reddit that one reason why MPX worked with bonus categories is because even though the purchase is via MPX/United, they put the correct category in which triggers the bonus.

The question is (haven't checked for an answer yet) is since this quarter is Amazon, a specific merchant and not a category, how it will work since the category bonus is AMAZON and not category based (like restaurants/fast food, which should transmit correctly). In other words, not sure if MPX simply transmitting a category of 'Bookstores' or whatever Amazon is will trigger the bonus points.