I'm not familiar with that dealer but I am with buying out of Califunny.
Before you can register you have to have the truck weighed and get a weight certificate from the scales.
Then you go to DMV, they classify the truck based on the weight and charge you accordingly plus you pay the sales tax.
The out-of-state dealer that I have dealt with refers buyers to a California agency that helps you through the process.
Most buyers lighten the truck, including the spare tire and only 5 gals or so of fuel, before they get weighed. If over xx lbs the DMV will put you in a class that has a much higher registration fee.
You can go to DMV's website and it will tell you more including the charges for each weight category.