Phone upgrade exchange packaging not included

I upgraded to a new device and in the same box that it come in usually there is a return sleeve for my old phone to be send back as my trade in. I did get a return label... just not a return sleeve. Can I just put my old phone in the original phone box it come in and put that in a box and stick the return label on there and send it back? Or does it HAVE to be put in the small white see through sleeve that is usually sent? I would rather go ahead and stick it in the old original phone box I have kept from when i first purchased it and send it back in that.. I think that would be most secure to be honest. I mean after all,  my new phones was sent to me in those same boxes. Please help me so I know that when I send this one back, that I will not be voided of the deal for my device upgrade/trade in.