As an alternative I would recommend showing a "+0.50$" bit when the amount entered is $0 or the difference if it is below $0.50. A tool tip could explain the "fee" - it'd be much friendlier than a pop up message.
If he could afford it, it would probably be best to either drop the fees altogether for prices of <= $1, so free games can honor their price, or otherwise set the minimum price to $1, so that the store at least doesn't collect more money than the dev himself.
