I've always known that just billing people works. I've heard stories of companies that pay thousand dollar invoices without verifying that any product or service was actually received or ordered. A company I worked for paid a yellow page invoice in the thousands of dollars for a second rate yellow page publication with little circulation. This yellow page publication simply sent everybody int he major yellow page an invoice to get their ads. They made it look like a renewal, but in fact it was for new advertising. 

For the last couple years, I've been billing people monthly for a service. Many of them used the service the first month (or day) and have never returned. Yet every month they pay the $5 to $29 subscription fee. I kinda feel like I'm cheating them. The truth is that they'd rather keep paying the monthly fee rather than spend the 5 minutes required to cancel the subscription. True laziness. Recurring billing is where it's at.