No contract, no problem you did your part. Shelby's deal is very fair and you don't need to worry about the car your buying. 13,000 miles is nothing for the GT.
And therein lies the rub. They're not rare cars, but "fair market" varies by many thousands, so good luck with the not a penny more part.
Since you want to drive it, look for a car with more miles on it and save $25k or more.
I thought I paid too much for mine. Now I think it was the best car deal ever.