I'd guess you opened a "non receipt" case when you should have opened a "not as described case" ["ordered 16 and got 8"]. Seller can show delivery ["of something"] so wins the case.
If that is the case then you can now open an "NAD" case [as long as within 30 days]. Just open the case - no discussion with seller - and seller should send label to return the 8. If he does not send label then escalate after 3 days. If he does send label return item and you should get a refund
@momofallboys