MailChimp offers two methods of buying email delivery credits when using our system.
Monthly Plans
Our monthly plan is designed for folks that send at least one campaign per month on a regular schedule. Monthly billing is based on the total number of recipients saved across all lists under an account. It's not based on how many emails are sent in one month under an account. Unused monthly credits do not roll over. Billing for the monthly plan is automatic on the same day each month according to your start date.
Pay As You Go Plans
Our pay as you go plan is designed for folks that send less frequently then once a month. Pay as you go plans allow you to purchase credits as needed, like a roll of stamps. Pay as you go email credits roll over from month to month and they do not expire.
Note
Any account that adds AlterEgo will get a 2% discount just for being security conscious. Not a bad deal.