Xbox 360 group product manager Aaron Greenberg was asked why you can’t pay exact amounts for content on Xbox Live Arcade. His answer clears up any confusion you might have:
“The reason why we do that, the core reason, is around credit card transaction fees. If we do this in bulk, we don’t have to burden the consumer with the transaction fees, or ourselves or publishers. It’s about keeping infrastructure costs down and I know sometimes it’s frustrating because you end up with odd points, but we don’t have any plans to change that,” quotes 1UP.