Chargebacks
Definitely write the credit card company. I work for a retailer and deal with credit card charges on a daily basis. Every merchant has a chargeback account, and if a customer writes and says they were wrongly charged, the money is taken from the merchant's chargeback account, and the merchant has a certain amount of time to prove they had a right to the funds (a register receipt, AVS, something).
Credit card companies do NOT like chargebacks... they will jump a merchant's tail that racks up lots of them... sometimes even more effective than a BBB call... get their credit processing revoked and they'll never rip anyone off again.