Is the $19 Subscribe by Email fee a one-time thing?

Just wondering. I purchased it this morning and installed it. Then I got to wondering if I had read the docs correctly. Is the $19 a one-time fee or monthly? I thought I saw something that said we could cancel after one month and keep the plug in. Does that mean I can continue to use it?

The blog on which this is installed is not of sufficient volume to merit a monthly charge. If it is monthly, I will need to cancel and get a refund.

Thanks.