Sorry if this is old news - but it wasn't to me. These are the credits you earn when you choose "no rush shipping" at Amazon checkout, offered for many items.
"You can use your No-Rush rewards towards purchasing a variety of things. Rewards can be used for buying eBooks for your Kindle, movies and TV shows on Prime Video, groceries and daily essentials on AmazonFresh
, and for 2-hour deliveries with Prime Now. You'll see which No-Rush reward your order qualifies for in the offer at checkout. Rewards are automatically added to your account once your FREE No-Rush order ships, and are automatically redeemed on qualifying orders. This is just another perk of being a Prime member!"