I would first see what they are going to spray. I went to walmart and bought 3m ruberized undercoating at about $5/ can and sprayed 10 or so cans under mine. I am sure I was much more meticulous than the dealership could ever be. If I was doing it again, I would spray sound deadening on the underside. They make a product that is sound deadening and an undercoat. I believe you can roll it on or they will sell gun that you can use with a compressor.
2002 F-250, PSD 6-speed, CC, short bed, 4X4, Toreador Red w/ Silver accent, A.R.E mx cap, Weld Super Single II and 33s in the summer, x-springs, F-350 blocks and badges, AIC, proflaps, ITP intank mod, AirDog with 3 micron filter, isspro guages and TTM, scangauge, husky liners, alarm, xm stereo,stull 5399-p grille, centramatics, ccv mod, AIS Air Filter, Southbend CON O, Detroit Truetrac LSD, coolant filter, 203 stat with billet housing, blackcloud intake heater plug/boost fitting, reese titan hitch, fabcraft PTO cover/temp sender port, prodigy brake controller, ITP Boost Annihalator, oilguard bypass filter, Mag-Hytec rear dif, bilsteins, hellwig rear stabilizer bar, ATS housing, non-ebpv pedistal and exhaust flange, magnaflow 3.5dp exhaust, ITP reg return, turbomaster wastegate controller, and ceramic coated bellowed up pipes, PHP 50t and 80p, currently 105,000 miles.