Who thinks that the post office is a socialist mechanism?

It's a service that costs nothing to the recipient (except for those who choose to rent PO boxes), but the user (sender) does pay for. About the only thing remotely socialist about it is the universal service mandate that was established by the US Constitution. I do not consider this socialist, by the way. Everyone pays the same rate for 1st class mail, and gets as close to the same service as the system permits. Socialism does not treat everyone equally - those who contribute little or nothing often receive more benefits than those who contribute the most.

Private carriers do not have to deliver to everyone, and can pick and choose what they will take and where they will take it, to maximize profit. USPS cannot do that, and those in favor of privatizing 1st class mail delivery (there is already competition for other classes of mail, and package delivery) fail to see that private companies would scoop the cream off the top, that is, take only the delivery territory that will make them a profit. There would still be US Mail, and the hit the taxpayer would take for it would make today's deficits look like chump change.

It is a quasi-governmental entity that is far from socialist. It primarily subsidizes businesses which advertise and send and collect checks by mail. The ancient Romans had a free messenger service.