What upgrades have you done to warrant a custom tune?
For them to "custom tune" your car they are going to have to burn a chip for it. Our cars are not like the newer OBDII cars which have hand held tuners to adjust certain parameters.
From your sig it is not worth the custom tune as your computer should be able to handle what you got.
2014 V6 Deep Impact Blue Mustang 2.73 (Uughh)
1989 Sonic Blue Mustang LX Coupe, 349 ci, TW heads, TFS Track Heat intake, 75mm Accufab TB, 80mm ProM, 30 lb inj., Jay Allen cam, TKO, 3.73, 31 spline w/ TruTrac, MAC 1-5/8" LT headers w/ h-pipe, 2-1/2 flowmaster, electric fan, Moates QH
392 rwhp 424 rwtq....Mustang Dyno
"There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves." -Will Rogers-