You are out of luck with those credits. If less than $100 (or $75, maybe, see next paragraph) you are stuck.
However, FYI I called to extend one recently. They said it would be $100 to extend, and I commented, "I thought it was $75", and they charged me only $75, but said that they would not be able to extend that courtesy much longer.... But IF you could get that deal, you could get a $10 credit at least, maybe worth a try.