"What taxes the USPS pays is a matter that states could raise with the federal government through their congressional delegations."
As a federal bureau, the USPS usually cannot be taxed except by the federal government. All Post Offices are officially US Government property (thus why state and local police cannot normally police a post office, the feds see to that themselves) so become exempt from state property taxes. I think refueling is usually done on-site with the fuel obtained by a federal supply chain so again the states don't get fuel taxes. As federal employees, state income taxes are questionable (depends on the laws), so state revenues from post offices are limited (the same is true of military installations and other federally-owned property).