When upgrading your account select the total amount of data that you would like in your account. Please do not select the amount of data you want to add to your current plan.
For example, say you currently have a 200 GB plan but you are about to reach your limit. You would like to purchase another 100 GB for a total of 300 GB altogether. When upgrading, you should select the 300 GB plan from the drop-down menu, not 100 GB. Because you are choosing the total size of your plan, selecting the 100 GB plan will reduce your total space to 100 GB instead of adding an additional 100 GB.