Sometimes billing errors are just that, errors. Calling customer support SHOULD get you a refund. I was erroneously billed once by MS, and unfortunately, it took multiple calls and 3-4 months to get my money back.
Worst case, tell them you'll issue a chargeback. Then, call your credit card company, and issue a chargeback for that charge. You get your money back, and the biller gets a small fine from Visa/MC/AMEX/Discover. When enough of these fines stack up, it's a VERY strong incentive for a company to fix billing "errors."