Filled my cart with more than my credit, expecting to pay the additional. When I tried to complete the order, I got a couple of red boxes with:
Something went wrong. Please try again.
1 error prohibited this record from being saved:
There were problems with the following fields:
Unable to pay for order using store credits
I removed some items from my cart until the total was under my credit amount, and the order processed correctly.
So I have my stuff ordered, but you might want to look into why that happened.