How do you put a CoD fee on for a player?

In your inventory there shout be an icon that looks like an envelope with an arrow on it. Click on a mushroom or cheap equipment and then click on that envelope icon. A small window will pop up with the name of the item you are sending and places to put the recipients username and the amount they must pay to get the item. This is normally how users send money because there is no direct way to exchange just money.

Thanks!