I recently canceled additional bandwidth as an extra with linode, and found what I would consider to be a flaw in their pro-rating system for bandwidth. I sent them this trouble ticket:

Hello, Just wanted to give you a heads up that I have a problem with the way you pro-rate bandwidth extras. If I pay for a partial month, and cancel midway through, I get a refund for the remaining part of the month. That makes sense. However, since I've still paid for the previous part of the month, that should be included in the calculation the remaining bandwidth allowance, but it is not. Its not a big deal, but since I'm a bit of a math nerd from time to time I figured I would mention this. Otherwise, love the service!