Yes, the way the site I used as example works, is the prices you set are minimum prices, but people can always input more. Setting the price to 0 means it's free but they still can give you money if they input something other than 0 in the amount box. Or you can put it for a minimum of 5, and they could also input more if they wanted to, but not less.
I had something selling at a minimum price of $5, and about 20% of the users gave me more, between $6 and $10. A few gave me $15... triple the amount I was selling for.