You need to go to the below link & read up because you need to set ad rates.
Once you set up your rates and it is a promoted listing that someone makes a purchase from then your billed the % of the rate you picked.
Then when ebay invoices you there should be a credit taking away from say my $30.00 , so you work down the credit over the quarter .
http://pages.ebay.com/seller-center/stores/promoted-listings/benefits.html#m22_tb_a11__4