After going back and forth with form letters a few times, I finally got a response directed at my actual question from the fine folks at iTMS tech support. Click on the link for more, if you’re interested in the outcome.
Their final reply:
Thank you for contacting iTunes Music Store Customer Service.
Because the credit card you have on file for your account is over its limit, the iTunes Music Store will not let you make purchases even though you have a store credit and regardless of the fact that the purchase you are attempting to make is well within the store credit limitations.
To resolve this issue, you will need to update your account information with an alternate, valid credit card. When the card you currently have on file becomes valid again, you can re-enter it as your method of payment for your iTunes Music Store account.
…and my final response:
Well, it’s good to have a final answer on this. Let me just say for the record that I think this is a poor implementation choice. I would like to make an upgrade suggestion to the store to allow purchases with store credit in such a situation, with the caveat that a check be performed to see whether it will exceed the amount of store credit. This seems like a relatively simple and reasonable thing to do, and could help alleviate much frustration in the future, if my personal frustration in this matter is any indication.