You need to call DHL customer service in your country - most likely international shippings number (as it may be different from domestic services number).
You can't do that online.
You must say AMD is paying for transport.
They will ask you dozen of questions related to shipping.
After that you will receive your shipping info on email.
Then you need to update missing information.
After this is done you can proceed with getting DHL to pick your parcel.
Value = cost of your CPU.
Weight = parcel weight.
Thank you. It sounds like quite a hassle to go through, but I'll try it nonetheless!
Also, the CPU came with a heatsink+cooler, but I was instructed to send back the CPU without them (makes sense) - do I have to take that into account in the value declaration? And if so - what is the value of the bundled heatsink+cooler that comes with the Ryzen 1600?
Are you from EU? If yes you don't need to fill customs declaration. When you will call DHL ask about that.
Just put retail price - it does not matter. You will get full box from AMD back - so your current heatsink is a gift
Make sure to pack CPU safely - use original plastic CPU holder, wrap it in thick layer of bubble foil / styrofoam, put it all in thick cardbox.
Don't forget to put your address and RMA number on the package.
And then be patient!
I was still in "fast" group so it took one week between DHL pick my CPU and bring new one. I know others had to wait 10 or 14 days. It looks process takes longer last days.