Ask any regular Xbox Live downloader what their biggest hassle with the service is, and they'll most likely tell you it's the inability to buy things on the service with "exact change". Instead, you have to buy your points in bulk, which after transactions often leaves you with a bunch of "loose change". Ever wonder just why that is? Xbox Live's Aaron Greenberg:
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.
But...what about...ack, we hate it when these answers come back and actually make sense. Why Can't I Pay Exact Change on XBL? [1UP]