Commissions on Ad spends are calculated at the end of a month, the commissions (% 0f Ad spends) are charged bassed on the actual spends.
For example: If you have subscribed to the $125 plan ($125 + 9% of Ad spends) and spend $2000 as Ad spends during a month. $125 is prepaid at the end of the month you will be billed 9% of $2000.