Be sure to check if you have any Amazon slow-shipping promotional credits expiring today. While there's no way to see when your credits expire, you can check your promotional balance
here.
You can also search your E-mail for the subject
"benefit conformation for your order" and try matching the dates in those up with what your balance is. I found I had only $1 in credit, but my most recent E-mail was for one expiring in July. I apparently used the ones expiring today already.