Not overly knowledgeable on Swedish law, but the idea that a public sector union can decide to block public services (in this case postal delivery of license plates) for a single company as a sympathy strike seems pretty nuts to me. Pretty sure Sweden would lose in European court at least (of course it would probably take a long time to percolate there).
PostNord is a private business owned by Sweden and Denmark. Unless there are specific laws to the contrary, its employees have the same right to strike as any other private sector employees. (More generally, postal services in the EU tend to be structured as businesses these days, because they are in direct competition with various courier and package delivery services.) Also, while the employees may be on strike, ma…
You have to get very high in the hierarchy before you find a manager that the unions will not represent. This is one of the major problems with the Nordic model I would say.