I found an interesting discussion on credit card use at Amazon.

In the main, they found similar to us. If a purchase is from regular Amazon, it will go through as "Category: Merchandise & Supplies - Department Stores". If it is from a Marketplace vendor, it goes through as "Category: Merchandise & Supplies - Internet Purchase".

However, if it's via Amazon, but is a mixture of Marketplace and Amazon, it also goes through as "Category: Merchandise & Supplies - Internet Purchase". In theory, this should make regular Amazon purchases give you the current 3x MR return. Maybe it's worth mixing it up a little if you have a large purchase?

I have not confirmed this.

Best wishes,

-Karl