First of all, you need to determine if it is bent or not. On some bikes, the seat stays are not symetrical. I just looked at my 2003 steel isis, and I would say the seat stay on the casette side is wider than on the non-casette side. Does the wheel move freely now that it is mounted? Does it rub anything?

If it is bent, you need to deal with the seller and/or shipper.