I'd call back one more time with "I'd hate to have to do a chargeback on my card, because this is a pain for both of us, so hopefully you will be able to have this sorted by [3-4 days from when you call]. I should have insisted at the time of the procedure since I knew this was covered, as I mentioned at the time, but I was too groggy. Let me know if you need any more information; otherwise, I'll look for that credit by [deadline day]."