You are absolutely correct to state that this must have been incorrectly graded, and although it is very late, I would like to take this opportunity to apologize for the inconvenience that this caused. I only wish that you'd had my contact details at the time, as I could certainly have got involved.
There has been a change in personnel as well as in procedures in the used department, consequently I would hope that nothing like this could happen in the future - and if it does, I would always urge any customers affected by problems like this to contact me directly to act on your behalf.