If it hasn't arrived, you are entitled to ask them to send either it or a replacement out. You could then return the replacement for a refund.
Essentially your contract is with the retailer and that is who you complain to. The carrier is working for them so the retailer so the retailer has potential redress against the carrier - but that is their concern, you bought from the retailer and how they get it to you is up to them.
They may ask the carrier to give them the scan of the signature and ask you to confirm it is not yours, or anyone known to you - but that is them sorting out their own internal processes.