We bill every Monday for what you spent in the previous 7 days. Campaigns are pre-paid, so when you first create a campaign, you will be billed once for the weekly budget. Each Monday after that, you are just billed for exactly what you spend. In essence you are "refilling" the budget each week.
Example: If you create a campaign with a $100 weekly budget, you'll be charged $100. If you spend $65 of the budget that week, you'll be billed just $65 on Monday when the campaign budget refills. If you spent just $20 of the budget, you'll be billed just $20 when the budget refills on Monday.
All campaign refill charges are aggregated into one single charge. You will see "Perfect Audience" on your credit card statement as the biller ID.