It would be nice to the buyer if the mushrooms came attached to rocks, but you can ship them loose and let the buyer attach them themselves. You can peel them off the rock by using a razor blade, then put them in a shallow tupperware container with rock rubble in the tank, and give them a couple weeks to attach to the rubble. Use a piece of bridal veil netting attached to the container with a rubber band if they want to float away.
Mushrooms are very hardy and should ship easily. They need to be packed in water, and most people include a hand warming heating pad thing from Walmart or the hardware store to keep the water warm. That may not be necessary depending on the time of year you are shipping them, and the destination. It's a good idea to ship them in an insulated package, like a styrofoam container.
It would be more profitable for you to sell them locally. Packaging and shipping corals is not cheap -- it may not be worth it for mushrooms.
