Those fungi spores that they are selling are primarily for eating, so they get to charge more for their goods. I'm assuming you just want this stuff to decompose on its own. There are probably enough fungi spores in the air to do it on its own.
Fungus needs 3 things to thrive - food, water, and air. The wood is the food, and since its above ground, you would have the air. But, you may have to keep things kind of moist to get the fungus going. If the wood has dried out, you may need something on there that would keep it moist. Chips, compost or dirt would probably fit the bill.