Corey, If you find that you are going over you data allowance on a regular basis, buying tokens is a less cost effective way to get more data. Data prices/GB are significantly cheaper if you upgrade your plan vs buying tokens.
All token purchases are billed to your account. They are also not used right away. Once you have bought tokens they stay in your token bank until you go into your account and "use" them. I hope this helps. If you have anymore questions or concerns feel free to let us know.