You can schedule an appointment for convenience's sake, but walk-ins are acceptable if the timing of your breakdown is... less than ideal. The locations will handle both in- and out-of-warranty repairs. And don't despair right away if there's no outlet near you on day one: roughly 200 more uBreakiFix locations should offer Samsung-authorized fixes by early 2019.
This won't help if you're using a budget Samsung phone, and it still means going to a third-party shop. Even so, it's hard to complain too loudly. There are few things more painful in phone ownership than having to mail in your device or put your trust in unofficial repair shops. This guarantees that you can avoid those hassles and spend more time actually using your phone instead of fretting over its fate.