Before you give up on them I'd also suggest calling them. Why? Well, I'm a firm believer in the 'personal touch' and a phone call is more personal than an email. Plus, you can also explain that you really don't need the numbers/logos any larger than say 7" wide at 72dpi. Given those attributes it's clear that you aren't looking to use their work on a 'real' car.
Firstly, the number wouldn't be big enough and at 72dpi (what most, I believe, the late model templates' resolution is) wouldn't be good enough to make a digital print from. It might help to reassure them that you aren't out to steal their work. Getting someone's name and offering to send them some examples of your work (maybe some of those rendering you have made?) couldn't hurt either. It's that reassurance thing again.
Just my thoughts.
