I just looked at Marti's website. After selecting "Pantera", it asks for an eleven digit VIN. I entered my Pantera's eleven digit VIN, added it to the cart, and proceeded to check out. Not sure what happened with your attempt.
Note that when asking for the VIN, there is a statement that says that if you have a car, your VIN should start with a number; if a truck, it starts with a letter. That statement doesn't appear to apply to a Pantera.