Sean- Just a tip for you so you will not be caught off guard on your $300 overcharge.
Contact your CC bank immediately and advise them that you have requested this correction and they have not yet complied. You have to establish your dates to protect your rights under the Federal CC protection act. I believe you have 60 days and then your rights expire, 90 in Canada. That would be from the date of your first statement listing the incorrect charges. Just tell your CC bank that you have notified VOOM anf they have not issued the credit. This will trigger a chargeback with VOOM and they will be required to show proof that they had every right to charge you the amount stated or issue the credit. No!, if you used one of those VISA Check cards or a debit card rather than a credit card, then you have no legal protection at all. You will have to sue VOOM. One good reason to never use VISA check cards or debit cards to make purchases. (I believe the legislation to bring these cards under the CC protection act is still in committee)
Normally, I give a merchant 48 hours to correct an error on their own and tell them I will give that amount of time, no exceptions! After passing of 48Hours, I automatically notify the CC of a fraudulant charge. Don't try to be the nice guy here, you will lose. Just conduct yourself in a business like and firm manner.