The problem is that you made the plan change effective immediately. When decreasing your plan allowance always always make the plan change effective at the beginning of the next billing cycle. When increasing, make the changes effective at the beginning of the current billing cycle.

You should call customer service at 1-800-922-0204 from a landline or *611 from your phone and see if they can reverse the change for the current billing cycle back to the 18 GB plan and have the next billing cycle start with the 8 GB

Based on the data provided, your bill and data allowance looks like this

30 day billing cycle. Changed plans on day 20

20 days on 18 GB plan for a total allowance those days = 12 GB (18/30=0.6x20=12)

Partial credit on bill for the 10 days you already paid for the 18 GB plan

Backdated partial charge for 10 days on the 8 GB for the previous billing cycle and the full monthly charge for the 8 GB for the current billing cycle.

10 days on the 8 GB for an allowance of 2.6667 GB (8/30=0.26667x10=2.6667)

If you used more than 12 GB in the first plan or 2.6667 GB on the second plan, you will be charged overage fees in 1 GB blocks.