Flat rate pricing means that there are no recurring fees. You only pay once but you can send emails for the length of the flat rate period (e.g. 2 years).
For a single flat rate payment you receive a monthly allowance of email credits that refreshes every 30 days. This means that you pay one time but you keep getting your credits renewed every month.
After 2 years you can choose whether to renew again for another period or not. Use your email credits yourself or share them with your Team. Each credit can be used to send 1 email to 1 recipient. If you a newsletter to 800 recipients this uses 800 credits.