There are lots of free ad rotation programs available and even 3rd party hosted services.
Some of the free software you can install on your site even supports recurring billing via Paypal.
Don't fool yourself, many advertisers use Google Analytics which makes tracking referrals and ROI from your site a piece of cake and trivial to implement.
I took checks in the earlier years, and still have from time to time for really big corporate clients cutting checks for $15K+ for a campaign, but overall now it's pay BEFORE you play with my ads using a credit card because sometimes the checks bounce and you end up paying big bank fees or the client simply didn't like the conversion rate (not my problem, he made the ad) and doesn't want to pay or quibbles for discounts after the fact.
All prepaid ads in advance is the only way to fly IMO, especially with small accounts as they are worth the trouble to mess with otherwise.
Anyway, look around, lots of ad software and billing integration, you can easily make it all turnkey right off your site.