Do you ever buy things from Amazon.com and choose No-Rush Shipping for whatever promotional credit they’re offering at the time? Usually it’s either $1 on digital media (Kindle eBooks, TV shows, music, software, etc) or $5.99 shipping credit for Prime Pantry (these are less useful to me).
The credits are worthless unless you use them, and use them by their expiration dates. This link shows you how many credits you have left for digital media (Prime Pantry credits are not listed here), but it doesn’t show you expiration dates. Those expiration dates are in individual emails, one for each order you made. I make dozens of orders per month that I choose slow shipping on, so I’ve let many credits expire just because I wasn’t keeping track.