"You pay only for clicks you've actually received, so you are not billed in advance. We bill every 30 days or after a pre-set credit limit -- $50 - $500, determined by history in the program.
The initial credit limit of $50 is incrementally raised each time an account hits its credit limit before 30 days have ended. The credit limit is first raised to $200, then to $350, and then to $500. The amount billed may be slightly in excess of the credit limit if an account accrues clicks very quickly.
For advertisers who meet our criteria, we will consider increasing credit limits beyond $500. Although credit limits cannot be increased in all cases, we are happy to review all such requests."