+1 for a solution to at least allow people to buy/pay for multiple ads.
Other very popular sites allow users to submit their first ad, and when they go to pay, the user is asked if they want to buy more ads at that time. Then they can just return to the site and list the rest.
There is even a custom $ amount which can be used as a credit and applied any way the seller sees fit; that may work well when categories are priced differently, but also with fixed amount imo.