This site offers targeted local advertising. I charge a listing fee of $12.50 for limited duration ads and $25 for unlimited duration ads.
I am only interested in ads that are suitable for the the local community such as a local blog, a local business or an event taking place in town. I reject other ads.
If I approve the ad; I will create the ad placement and send an invoice through Paypal for the listing fee. The invoice includes a link showing details of the ad placement. Paying the bill implies agreement with the terms.
I consider the payment a "listing fee" and do not offer prorated refund. This is largely because the amount of money involved is quite small. Instead of refunds, I send an invoice: If you do not like the placement; do not pay the bill.
I run fixed duration ads until the expiration date or 40,000 page views, which ever comes first. I will display the long duration ads for 100,000 page views. I will remove ads if the target for the ad stops working.
100,000 is a huge number. 25¢ per thousand is a very low rate. The catch is that it might take several years to display a targeted ad 100,000 times.
Placements are not exclusive. Other businesses can buy targeting the same ad space. This will prolong the time it takes to display 100,000 ads.