Do I need an appointment just to return plates?
Not at all, it's when they have time available, and also depends on which dmv had the time. It took me 3 months to get an appointment. However I see that little by little more times are being available. Keep checking... And whatever you're going to do there, make sure you have all the documents, because if you forget one thing, they will send you home and you have to wait until there is another appointment available online before you could come back
You can only do it online and its only but certain slots.
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