I ran across a posting earlier tonight related to this that might help. I'm no expert because I am still trying to get this contribution installed.

Instead of entering 0.00 for the price, did you try leaving it blank? I saw a post where someone was trying to do the opposite of what you want and the problem was he/she left it blank and thought it would be the same as 0.00 but it kept listing as free.

Hope this helps.