If it’s been bought from Amazon via the WiiM Store, then Amazon only ship it, they’re not the seller so I guess that’s why they’re saying it’s not returnable - not returnable to them, but hopefully to WiiM. Actually, their product page says “Eligible for Return, Refund or Replacement within 30 days of receipt”…
Initial recourse is normally thru messaging the third party seller on Amazon and I’m surprised the webpage didn’t lead you thru that. It’s maybe worth another try.
Notwithstanding that, hopefully WiiM have picked up your ticket and will get in touch. It might be worthwhile updating the ticket with your order details. If you send me the ticket number, I’ll try and escalate it to WiiM on your behalf.