Check out the disAbilities FAQ thread, it's a sticky "tacked" at the top of this forum. There you will find a list of preferred vendors, as well as which vendors (preferred or not) DISers have recommended in the past.
Preferred vendors are allowed to utilize Bell Services at WDW resorts for drop-off/pick-up.
Non-preferred vendors are not necessarily "worse" -- they will meet you personally for drop-off/pick-up service. Some of these vendors are also highly recommended.
Enjoy your vacation!