I would like to...?

It might be safer and less of costly if they just mail it to the horse facility or something. Plus blankets and saddles and stuff can get pretty heavy and therefore cost a bundle to mail. Also maybe you should start out locally and see how well that goes first. Just put flyers around to donate (say where they can drop it off or put your number on there so they can call you to pick it up) or set up huge boxes in petstores for the donations. Then go by later and pick them up.