So if the Founder were libertarians, why would they want big government to run post offices? They wouldn't. They weren't.
How can you speak of "the Founders" as if they had a common set of beliefs? They did not. Consider, for example, the philosophical differences between George Mason and John Adams.
They were classical liberals, essentially libertarians. That does not mean they were libertarians. Also, the postal service was the country's comm link. There was no private service available, nor was it possible to create one of the same capacity. As a matter of fact, the service was necessary to conduct govm't operations and for the conduct of commerce, which was one of the major motivations for the creation of the Constitution in the first place.