The law (including ECHR) protecting the private property in the mail that is being trespassed by the workers refusing to either deliver or give it back, and the law setting the rates and service standards for the post which was paid and not honored.

I think the workers at Tesla should unionize and get better treatment. I wouldn't want to work for one of Musk's companies. My concern is that if I lived in such a country, I'd be afraid that one day my city water would be disconnected or they'd refuse to renew my drivers license or passport, because I did something that those workers didn't like.

I'll admit to not researching the details of PostNord, but it is listed as a state-owned enterprise in Wikipedia and it fulfills Sweden's participation in the UPU. If it were purely private like FedEx, I wouldn't have the same argument--though in that case, my argument would be that the DMV needs to use another post if they know one is refusing to deliver.