Time frame is tricky, they sent a invoice reminder only 2 weeks ago. Which I immediately replied of "what this all about" (I signed 2 months ago)
However it sounds like you've other services with them you have already paid for, and thus it's less likely a company would pursue an individual for monies owed but rather discontinue all services they have. Their path of convoluted or hidden / unclear fees is certainly an unwelcome surprise.
Usually services are provided only after the customer has paid. Those that don't are either incredibly generous and trusting ... or absolutely precious. In regard to precious, in my rule book for all business and personal purchases, they are deserving a close and critical eye with an expectation of a poor and bad outcome to surface, and being prepared with a contingency plan having already shopped around without being in a panic, should the moment arise. I think it's fair to say some smaller and newer companies providing services have tight spots as well, and generally resolve as the company grows.