For Prime members who choose slower shipping, Amazon offers a $1 digital credit. I know many people here used the program to buy eShop, Xbox, PSN, and Steam codes - those are no longer eligible items. I confirmed today with Amazon that the change took place at the end of May.
For myself, this was easily the best deal around - free games. But unfortunately this is no longer the case. So unless you're eager to snag those MP3s, time to stop using the No-Rush Rewards program.
First GCU and now this, these are truly the end times.
EDIT: For clarification, No Rush rewards earned before this change will still work (should read as expiring in September or before), those earned after (expiring in October) will no longer work.