The problem with certain types of libertarians (which the Koch brothers believe(d) they are) is that they overdo the market fundamentalism to the point of turning it into religion. At that point, anything that is not "of the market" turns into a religious war, with all the stupidity of all religious wars.
Markets are phenomenal things for the majority of goods and terrible things for a few goods (like common defence, the judiciary and pollution control). The post office is in the latter case - some people need to subsidize the system in order for (say) rural deliveries to take place.
The benefit of having a single universal system is important in a world where an enormous amount of things that are necessary to a functioning society (say, jury summons, or voting pamphlets or other government paperwork) need to be accessible to every single citizen in the democracy.
Eventually the Internet can replace most of these things, but then the government of a democracy needs to ensure that literally every citizen has free and reasonable access to the internet without meaningful burden, lest it disenfranchise the poor and the rural.
Oddly, the whole "defund the post office" thing seems to be mostly a Republican issue. I say oddly, because the Democratic base tend to be from more densely populated states and from larger cities in those states. These people are least likely to be affected by a lack of post office as commercial delivery services can supply the majority of their mailing needs in a pinch. The people most affected are people in large states like Texas, especially those outside of the cities, where it's less economical to deliver to.
Not to mention all of the other federal services that post office provide that would ultimately go away if the post office did. Things like passport processing, for example.
Honestly, I'm still kind of amazed at how people can be convinced to so consistently call for things that are against their own interests in the name of some abstract idea like "religion!" or "the market!.