The Constitution only “empowers Congress “To establish Post Offices and post Roads”. It does not say it must run the Post Office!
Sorry for shouting but it's clear that, without exception, private enterprise always beats a government-run entity. Look no further than public schools compared to private schools. Likewise, the artifice of the taxpayer-funded Postal Service stifles what would otherwise be a very efficient, cost-effective delivery system with the likes of FedEx and UPS going head-to-head in a free market to offer consumers real value.