SpamZilla runs on a PAYG (Pay-As-You-Go) system where users need to purchase credits to process domains. Credit packages are a one off payment and NOT a subscription. Also the credits you purchase will never expire.
1 domain = 1 credit
If the response from SpamZilla is an 'Error' you will not be charged a credit for that domain.
SpamZilla stores the domains you've previously processed, you will not be charged extra when processing the same domains multiple times.
Select the credits option in the profile menu on the upper right hand side of the screen.
From the below page, select one of the three credit packages to purchase.