Each month, on the date of your billing we charge for the upcoming month - this is based on if you have selected our unlimited plan at $12 per month.
In the cases where you upgrade a user's plan or add a new user during your billing cycle, then the next month you'll get charged for both the new user's upcoming month as well as a one-time prorate charge based on the number of days the new user (or upgraded plan) existed in the previous billing month.
If you remove a user mid-month and then add another user of the same plan type, there won't be any additional charges.
However if you remove a user mid--month without making any other upgrades or additions in the same billing period, you won't get any credit back. Adding a user and then removing them a few days later (within the same billing period) will result in a full month's charge for that user.
You may also have selected an annual plan to save money at which $X per user per month (billed annually) will be processed from your credit/debit card.