Annual vs. One Time Payment Options (Directory Plugin)

I'm checking out the directory plugin, and I'm confused about the payment options. The typical scenario I picture for a directory is a Yellow Pages clone, where you would pay an annual fee for one listing on the service (and an additional fee for each additional listing). Is that how this works? If that's the case, what does the one-time fee do? Is there any way to turn off the one-time option?