The salesman is right if it's got the tow package, it will come with a tranny cooler. Your better off running them in line. You can tap off of the line coming out of the radiator and install your aftermarket cooler. The cooler in the radiator will work more efficiently(liquid cooled as opposed to air cooled). Then your aftermarket(larger) cooler will do the rest. Get a trans temp gauge to monitor. Ideal temps would be around 175.

Shipping shouldn't be bad. No more than $10 I would think through UPS ground. Let me know.