Amazon has an program to buy in bulk and manage distribution. You can charge to a card or bank account. As others have mentioned, my first concern would be getting the credit limit to handle the volume. From there it probably needs to be multiple cards - i.e. max out the Amex Business Amex annual reward limit, then rotate in whatever else you need/want (i.e. can you spend your way to DL Diamond)? Or perhaps get multiple Amazon cards - i.e. the Biz Amex and one of the personal Visas.

Maybe in year 1 take a look at the Discover It business for some of the spend -- 1.5% cash back but 2x (effective 3%) first year.