trade in using return label

Hi, 

I am one of those who mistakenly thought the return label that came with my new iPhone 12 was the label for the trade-in shipment. 

I sent my iPhone X with the return label in October. I was contacted to ask for the IMEI number of the phone, which I provided in November. I still haven't gotten a response and I keep paying the full price for my new phone (haven't gotten the $800 credit and don't know I ever will).