To clarify, if you elected to trade-in your existing device during the online purchase, the shipping kit required to return your existing device will be sent separately.
The shipping kit, to return the trade-in, will be timed to arrive soon after (within days) of receiving the new device. This is done such that you will have time and opportunity to transfer, should you choose to do so, all data from your old to new device.
When the transfer has been successfully completed, you can prepare the old device for return to Apple’s trade-in partner. You must prepare the old device as follows
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
Ensure that you follow the return instructions precisely. The shipping kit will include:
- return packaging/shipping container
- a return (shipping paid) return label
- instruction leaflet
These items will likely be sent to you in a plastic shipping envelope; be sure to check that all items are accounted for before discarding the outer wrapping.