Do you charge for updates or new features?

No. Your plan includes whatever it includes, and new features arrive in it without a charge. The price you agreed is fixed at the moment you chose the plan, and a later price change never rewrites it. What does cost extra is separate and named: the POS add-on, a yearly price shown under Settings → Plan & billing, and a domain of your own if your plan does not include one. Anything outside your plan is shown with a small lock rather than hidden, so you can see what an upgrade buys before paying. The POS app on your tablets tells you when a new version is ready.