Common errors are:

1. If the shipping zip code is different than the bill to zip code for the credit card, then separate addresses need entered correctly. On the cart page above the selection for shipping, enter the ship to zip code. On the checkout page, the left side is for credit card billing information. On the right side, the shipping information. If any of this info is mixed up, the shopping cart will require that you return to the cart and correct the zip code.
2. Terms and Conditions check box needs checked.
3. Cart Error:
Process Status: 521
Status Message: Error. The Orbital Gateway has received a badly formatted message. Field [AVS CITY] exceeded max length of ²⁰
The problem is you exceeded the length of the field. Try abbrevating whatever field has too many characters. For example, address, city, or name. If it is your address you can use address 1 and 2 fields.