you typically have two choices. One, ship to a post office, and walk out/hitch out to pick it up. you pay the cost of postage. Two, you ship to a third party ie MTR and you pay for postage, and the "handling" fee for them to manage it for you. You will have to go online to check for specific details for each place. This year we are shipping food supplies to Red's Meadows, for $35, and Muir Trail Ranch for $55 (plus postage) . We also have some friends coming in at Independence to re-supply us for free! Couple of years ago I sent my re-supply to Mammoth Post Office for the flat rate priority of $15.95 I think? then I walked out/bussed out from Reds to pick it up.
So the extremes are kinda like from the flat rate priority box, to a reserved horse packer, and can run from $15 all the way to $300. YMMV