During his reign as Governor, "Dollar Billy" Clinton floated a plan to collect Arkansas sales tax on items bought outside the state.
Clinton's plan basically involved collecting all packages coming into the state, opening them, and seeing if they contained taxable merchandise.
He ran slam into some unexpected opposition - Marvin Runyon, Postmaster General. Runyon informed Clinton that his plan of opening and inspecting packages coming in through the US Mail would be a Federal crime, and he (Runyon) had armed postal inspectors who would be authorized to use deadly force to ensure that the mail went through. Clinton backed down. My Dad was one of those inspectors at the time.
People like to rag on the USPS, but they never hear of the things they do right.