Are these designs you created that they are wanting or 3500 shirts? I have never sold t-shirt designs because I'm new to the business, but I was a pro photographer. If anyone wanted the copyright, negatives, etc., of the images I took, the price was always exorbitant. I needed to make sure that any profit I lost in selling the prints was made up in selling the copyright. I would assume that selling any designs would be the same. Good luck figuring it all out.