[Elsdebo]

If they have no proof, normaly they  have to prove that you resived it. I would try to contakt them, maybe say that you get a laywer, because you are praktikaly in the right, did they tell some thing about a investigation within the deliverer?, this would normaly the first stepp for them.

Dont say you get a lawyer. You want to avoid playing a legal game. IF they call that bluff, your quickly lose credibility.
Indeed, they have no proof. They can only bully you. Let them waste time and resources. As long as you don't get official letter you are fine.
And even when you get one, you're still a long way from being in trouble. Altough it is not good for opsec if its your real name.
Indicate you are willing to cooperate in some way, but let them take all initiatives.