I am a UPS man. What do Libertarians think of labor unions?
I think as long as there is no law requiring a person to belong to a union if they work at a particular job, or a law requiring an employer to hire a non-union person, even if they only want to hire union people (which is my understanding of how "right-to-work" states work) then I don't see what the problem is, because at that point people are just interacting on a voluntary basis.
On a personal note, I think unions can sometimes serve a purpose (especially in the past during the Industrial Revolution to get better working conditions) but I think unions can be bad sometimes. From what I see (my father-in-law belongs to a union) they seem to come from the point of view that all workers are equal and should get equal pay. What this translates to in practice is that often a lazy worker will get paid the same as an industrious worker -- there's not much incentive to work harder.