Previously, you could bank these up and buy a $ 20 XBOX gift card (that can be redeemed for physical items sold by Microsoft Store). Last week I noticed that only my Sept 1st expiring credits were applying and I noticed a change in T&C:
New (Oct 15th EXP credits)
https://www.amazon.com/gp/promoti…G2PG2
Note the line: "Offer only applies to products sold by Amazon.com or digital content sold by Amazon Digital Services, Inc. (other than limited digital content). Rewards may not be used for the purchase of software subscriptions, gift cards, digital video games or memberships."