I apologize for any confusion there may be with the billing on your account. Charges on a Constant Contact account is based on the your amount of contacts and the email plan you are under. Plans for the pricing amount you mentioned are accounts with 2501-5,000 contacts. If you have less than this amount of contacts you will be charged less. While if you have more, your billing price will rise as the charges are all based on the amount of contacts at that time.
With that said I can see you have also spoken to our Chat Support about this as well. This agent has submitted a ticket to our Billing Support on your behalf. One of these agents will contact you directly as we are unable to go into your billing details here in the Community.