I was just told, via chat, that the credits could be used for Amazon Fresh product. (he wouldn't do a promotional credit). Haven't tried it yet though on Amazon Fresh. Here's the text:
I'm sorry but we don't have an option to do that <<NAME>> we cannot convert it, those reward you earning is eligible for those select digital products, including instant video downloads, Kindle e,eBooks, Digital Music, and Amazon Appstore apps, as well as on AmazonFresh and Prime Now. Rewards are automatically applied at checkout.