I have Am Prime. Occasionally, on multi-item orders, they ‘offer’ a couple of dollars ‘digital’ credit if I allow extra days to combine shipping on items.
The problem is that the digital credits expire after so many days, so they are basically useless.
you do have to eye the expiration date. i am using one credit today to knock down rental fee on a movie. i am happy to play this game when i don’t need quickest of deliveries.