What method have you used to contact Princess? We had an issue like this several years ago and found that you cannot deal with them over the phone. Put your complaint in writing... over and over and over.
Don't wait weeks, if you don't have resolve after one week, write to them again with a clear and concise explanation of the overcharge and tell them you expect this to be cleared up. THEN follow up with a phone call.
Document everything. Track the date and time of every contact as well as the name of the customer service rep.
If after 2 or 3 attempts, let them know that you will get the Better Business Bureau involved. They will eventually give you the refund, but let them know that this is costing YOU money and tack on the interest charges.