We had never charged for it after the sale until about 6 months ago due to people calling all the time for driver installs, scanning issues related to their internet etc.
We used to offer an optional network/it yearly agreement and people would decline and then start calling for help. We decided to send out, initially an opt-out to ever customer who had current devices, and after every sale send one out. We did it as an opt out due to the fact that customers would decline the option and start calling. We charge 20.00 per device per month and once we started doing it about 75% of the customers kept it and the other 25% if they want help pay $130.00 per hour fees. It is a nice revenue stream (thousands a month) and we make sure they know we will be there no matter the problem. It has worked well for us and is easy to manage.