Pricing is based on how many email messages you send per month. If you send out one email campaign to a list of 1,000 contacts, that is 1,000 messages. If you send out 2 email campaigns to the same list of 1,000 contacts, that is 2,000 messages.
With ResultsMail you sign up for a monthly volume plan based on your expected monthly usage. These plans may be viewed here. You are then billed on a month to month basis at that rate until you either change it by requesting an upgrade or downgrade, or cancel your account.
Please note that at each level there is a small overage charge per email sent over the limit you sign up for. If you send significantly more than the level you sign up at it would make sense to upgrade. Upgrades will be made effective immediately upon receipt of the request and are retroactive to include the entire billing cycle during which the upgrade was requested.