Pack your smartphone in a cardboard box.
Pack your smartphone in a cardboard box for optimum protection. If you don't have a cardboard box, try your local shop! They will surely have what you are searching for. :)
Keep your accessories (cable, wall socket, case, earphones) and don't send them back with the smartphone.
Wrap it so that it cannot move.
To prevent your smartphone from moving around inside your box, the idea is to pad the empty spaces with newspaper, for example, or bubble wrap if you have some!
Close the box and stick the voucher.
To close your box, use large removal tape to prevent it from opening during transport.
Finally, stick your voucher on the box so that it is visible to the delivery person.
Check, post and off you go!
Last check, shake your box (gently! Don't break it ;)): if you don't hear your smartphone moving around inside it, your parcel is ready to be sent.
Drop it off at the carrier Office, and they'll take care of the rest!