The price of the monthly plan is calculated based on the number of unique subscribers to whom you send email within a month. For example, if you have 1000 active subscribers in your database and you send some messages just to 500 subscribers within a month, you will only pay for that 500 subscribers.
The Pay As You Go works like a credit system. You prepay the credit that is charged to your account. 1 credit is then withdrawn from your account for each email message you send out to your subscribers. For example, if you send 1000 emails to 500 subscribers, it will cost you 1000 credits.
You've selected prepaid credits for .