Empty box to put the trade-in into arrived but there was no return label.

In this case the label is a single thin sheet firmly stuck to the cardboard fibre of the box. It cannot be lifted. I think my best bet is to physically take it into an Apple store. Thanks for your reply. It was worth a shot. I can't believe that there are so many Apple customers who are having this problem and it is still the way Trade-ins are handled. Apple are not learning from these experiences. Nice box for free though.

No it's not in my area. It is a DHL delivery and pick-up. The address label to me from them is firmly stuck down to the deep fibre of the cardboard box and cannot be peeled up - although I have seen that type of label before. So I think it may be area specific.

Managed to contact Apple after about five hours. They told me that the Courier would have a label and attach it as he/she picked it up. 

My advice is to photograph or video that transaction/exchange.