Sorry to hear about the unexpected overage fees and confusion over Carryover data, maniclems. I'm happy to help clear this up. You will first go through your allotment of data in your monthly plan and after that is expended, you'll use your allotment of Carryover Data (if any is available from the prior bill cycle). See #6 of this article for details: http://spr.ly/66041GaZO You will only receive an overage if everything is used. Here are some tools for monitoring your usage: http://spr.ly/66051GaZP Also, you might consider Safety Mode which slows down your mobile data once you have used all available data but prevents overages. Here's more on that: http://spr.ly/66061GaZu
See #3 for how to enable Safety Mode. Does this help out?
How does it make sense for my account to be hit with data overages and still tell me I have carryover data? Why isn't the carryover data being used before the overage charges kick in?
Is there any carry over data? If there were, you would start a new bill cycle with your base plan, plus whatever you didn't use of your base plan last cycle. Carry over only carries one month. So if you are trying to stash data for several months, don't. It's a buffer once in a while, but data does not roll continuously.