Pre-address one is OK. At the clounter clerk ad the over night label. Good luck.
If you are sending an envelope overnight, you should use the express envelope. It's free and available at the post office or online. But, if you don't have one, you still can print the label at home and use your own regular envelope. And also remember there is a cut off time for next day express, so I would recommend going into an post office to ship it.
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