From the Model Mayhem FAQ:

Why did I receive an "Access Denied" or “Error” message when I tried to submit my payment?
If you receive an unexplained error message when you try to upgrade while using a credit card, your transaction may have been declined unexpectedly. You can utilize an alternate payment method, such as PayPal to process your transaction right away, otherwise you will need to contact the customer support so we can investigate and help to resolve the issue for you.