Invoice is generated and charged against each account at the start of each calendar month for the previous month (after the fact).
The charges are calculated based on the plan selected for each site, pro-rated per day.
Example
If Account X had the following 2 sites and plans for January (which has 31 days)
The monthly charge for the account would be
($30) + ($120 x 16/31 ) = $91.94 (additional taxes may incur)