They can try suing but the postal service hasn’t done anything wrong. It’s not their fault that the government department in charge of issuing plates has made it a requirement for their outsourcing partners to use the national postal service to deliver plates.
To be clear, this isn’t an action, this is by default an inaction of a group in solidarity that’s having an unintended (and subjectively a somewhat funny side effect); but it’s the registry’s rules that are causing the impasse.