Your payment method is charged at the beginning of the billing cycle based on the estimated price level for that month.
If you exceed that price level at any point during the month, sending won't be interrupted.
Instead, a pro-rated charge will be calculated based on the difference between the price levels for the number of days when your contact list moved up into the next price level during that billing cycle.
You will see the pro-rated charge included on your next invoice, along with the charges for the upcoming month.
You can see pending pro-rated charges (if applicable) on your invoices page.
And you can monitor your active contact size on your billing page throughout the month.