return label for trade in

When you receive the trade-in kit from Apple, the return label is underneath the label on the top of the box with your address. You'll see a red arrow to peel the label off, which then becomes the return label.

Have you actually received the trade in kit? It comes a few days after you receive the iPhone and is a different box from what the new phone came in. And if you're in the US, it is delivered by Fedex, not UPS, which was the shipper for your new iPhone.