do I need to get a box from Apple to send in my device?

If you elected to trade-in an existing device during an online purchase from Apple's website, a shipping kit required to return your existing device will be sent to you.

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 adequate 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 may be sent to you in a plastic shipping envelope; be sure to check that all items are accounted for before discarding the outer wrapping. In some regions the shipping container will be mailed without an outer wrapping; in such instance, the return shipping label will be beneath the address label that was used to send the shipping box to you.