I am a seller and am disappointed that you find yourself in this situation.
In defence of your supplier all I can say is that he must have been hurt by abusive buyers and he has gone ultra defensive. As communication has taken a nasty turn I would stop communicating with seller.
I hope you had realised that package was tampered with when delivery took place and if you needed to sign then you made sure that this fact of tampering was noted on the delivery signature.
It may seem odd for a seller to give you this advice but you come across as a genuine honest person and if you were my buyer then I would accept your claim.
Please open a case on eBay qualification being that you ordered 2 Pieces and you only received 1.
I think correct term / type of case is Missing parts and you can look this up on resolution centre or types of cases.
I would not expect your Seller to accept this case and after a period case can be escalated and I would expect eBay to close the case in your favour with a refund and they may decide not to debit the seller. you should load the envelope showing tear on the case as evidence.
If you have not already then please send the seller photograph of actual package showing damage. This evidence will then be available for eBay's return team to view.
You don't need to contact postal service provider as they will only answer to Sender and not recipient, however if your seller makes a claim then they may contact you to confirm what happened.
I hope it works out to your satisfaction.
