Having the ability to do Window Tint is OK, but there are a lot of shops doing this. I would subcontract this work on having a really good installer come to your shop by appointment only. This way there is no overhead. You can get P.O.P from the companies that make it and sell the job with the installer knowing what tint is needed and the car it is going on. The same goes for Dent Removal.
Look for companies to send you display materials if you sell their stuff
. Try to coax as much FREE samples if your just trying to fill in empty space. Don't forget that your paying rent for floor space and every unused foot is costing you money that could have something to sell in its place. Even having air fresheners or key fobs next to the register is a way of picking up extra $$$.