Hello Christinasu520,
I understand that might seem counter-intuitive since post usually refers to after, however, with post-paid services monthly charges are for the upcoming month (see definitions here). I think the term might refer to receiving a monthly bill stating any accumulated additional charges during the month (see here). While the plan fee is charged prior to each month, overages and additional charges show after each month.
For any given billing cycle, you are charged the upcoming monthly fees as well as any overages you might have incurred during your previous month. So you are charged in advance for your monthly plan plus any usage above your allotments in the previous month.
While you would not be required to pay for another full month should you choose to change providers, there may be an amount required (prorated fees) depending on your billing cycle and the date of service cancellation. Generally, your billing cycle was determined when you activated services. The payment due date 21 days after the cycle end date. Even though your payment due date may not have passed, you could already partially be in the middle of a new billing cycle.
If you cancel services before your payment due date, you would receive a final bill outlining the partial charges required. On the other hand, if you paid the bill and chose to change providers before the end of the billing cycle, you would receive a credit for the unused portion of the billing cycle. That credit would get sent to the address they have on file.
If you remember from my reply to your other post, these forums are community-driven and not intended as a venue for customer services. We do not have access to customers' accounts. If you would like to discuss your bill, you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Cheers
