I recently bought a free game for $3 and really enjoyed it. So after finishing it I went back to the store page and wanted to get the bonus package for $10. Unfortunately the site offered no option to pay the remaining $7 and instead made me pay the full price again. Now this is was not a big enough issue to justify a refund (I wasn't even sure if that was a valid reason), but it left me with a bad feeling and I will probably not pay more than the minimum upfront again. I also now have two copies of the game in my collection with no easy way to get rid of one of them.
So please add an option to increase the initial payment amount without having to purchase another copy. I think Humble Bundle has done this quite well if you're looking for an example.