Stamps.com is an Internet postage provider service similar to a couple of others, such as, Endicium which does charge a monthly fee depending upon which plan a person selects. The Pro plan is $15.99 per month, their Premier plan costs $24.99 a month, and they do have a 3rd plan called the Enterprise plan which is advertised as being ideal for small businesses.
I'm a little puzzled though, as to just how you managed to end up enrolled in their Pro plan unless you somehow went through the enrollment process. You can purchase coils of stamps directly from the U.S. Postal Service on their website at usps.gov and there is no monthly service fee involved like there is with Stamps.com. They only charge for the stamps + shipping costs.
At this point...if you have not already done so...you need to contact Stamps.com to have them cancel any subscription to their service you may have inadvertently ended up on. I also have another phone number for them which I used to cancel a subscription I had with them. The number is 1-310-482-5800.
Hope this may be of some help.