So I e-mailed East Maryland the same afternoon as my visit, explaining that I was disappointed that the coupon I had given them was not honored, and they e-mailed me back before the end of the day, apologizing and letting me know that my Visa would be credited the amount of the coupon. 

Then they called today to see how the boys were all doing after their shots, and acknowledged the bill issue again, and hoped that everything had been settled to my satisfaction.

This kind of immediate and friendly follow-up redeemed them and gained them another star.