AVS stands for Address Verification System, which is used to determine if the billing address is correct. If the billing address listed with the credit card being charged does not match the one supplied by the client upon sign up, then this error will be thrown. If you receive a notice that your card could not be processed due to this error, go to View Account Info on the Control Panel Homepage and then select Plan & Billing Info. Make sure that the address listed matches the address that is associated with your credit card. Save the info and then notify firstname.lastname@example.org to have them try to run your card again.