The switch is in your account settings under tracking-link settings. It's off by default, and turning it on takes effect immediately on links you've already sent out. The button only appears once an order comes back marked "needs changes": your customer then sees a cancel option on the tracking page, confirms once, and the order moves to cancelled — anything already paid returns to your balance automatically, your balance and order list refresh right away, and our staff stop working on it. The page tells the customer the order won't be processed any further and that refunds are handled by the seller they bought from, so whether they get their money back remains your call. If one link covers several orders, those sitting in "needs changes" are cancelled together and the rest are left untouched.