Amazon is demonstrating some true Holiday spirit with deceptive promotional offerings. So deceptive their own support people don't understand it.
Watch out. Amazon will offer you $5.99 for Amazon Pantry items in exchange for accepting slower shipping options.
After 3 phone calls and nearly 1.5 hours on the phone, Amazon support was finally able to explain it.
It's only works for a very limited number of grocery items that Amazon sells directly themselves. You'll know because "the item will be marked as an Amazon Pantry item."
So I got two credits. About $10. I figured I would at least buy an item for free. I chose Vanilla extract for about $10. However, it was going to cost me $10 to get it. Doesn't make sense, right? Well, you can only use one credit at a time. So that means I can only use $5 to buy the $10 item. Then for any "pantry" items that Amazon sells you are charged $5 for a "boxing fee." Usually this is called shipping, and since I am an Amazon Prime Member, I expected my shipping would be free. Not the case. So in the end it would cost me $10 to get a $10 item as an Amazon Prime member, even with a $10 credit.