Awhile back I had a similar situation where my account was credited a sum of money in the hundreds of dollars. The money wasn't mine; it was clearly an error. You wouldn't believe the difficulty I had trying to give the money back! I spoke with multiple people at both the vendor who made the credit and my credit card company. They all insisted it was correct. Most of them didn't even want to investigate it! Finally, one of them told me they would look into in. At this point, I left it alone since I had spent more than enough time trying to give the $$ back. Six weeks later the error was corrected.

My suggestion is to call the vendor and explain the problem.....once. If they don't want to look into it, that's there problem.