The real issue is that the Constitution does not specify the MEANS of exercising the specified powers. Then there is the fact that the CC discussed limiting the fed/gov to the specified powers and it was rejected.
There are many more tasks charged in the Constitution to the fed/gov than just delivering the mail.
The delegates to the Constitutional Convention were actually on their guard with respect to policing against unwanted means when they drafted Congress’s constitutional Article I, Section 8-limited powers.
For example, delegate Benjamin Franklin had suggested adding canals for supporting commerce to Congress's Section 8, Clause 7 power to establish postal roads.
"Article I, Section 8, Clause 7: To establish Post Offices and post Roads [and Canals;]"
H O W E V E R …
Delegate Alexander Hamilton later scandalously ignored (imo) the following convention discussion concerning his unconstitutional national bank. He ignored that his fellow delegates had dropped Franklin’s suggestion for canals because some delegates felt that having such power would give Congress an excuse to establish a national bank as a means to supporting canals, delegates evidently objecting to predictable federal interference with INTRAstate banks.
“A proposition was made to them to authorize Congress to open canals, and an amendatory one to empower them to incorporate. But the whole was rejected, and one of the reasons for rejection urged in debate was, that then they would have a power to erect a bank, which would render the great cities, where there were prejudices and jealousies on the subject, adverse to the reception of the Constitution [emphasis added].” —Jefferson’s Opinion on the Constitutionality of a National Bank : 1791.
Consider that the early Supreme Court wrongly decided McCulloch v. Maryland McCulloch) in Congress's favor imo, that case testing so-called banking powers in Commerce Clause.
But such a mistake would not be surprising. This is because, about two years earlier, the rookie 14th Congress had misunderstood the Founding States intentions for the General Welfare Clause, (GWC; 1.8.1), Pres. Madison clarifying in his explanation for Bonus Bill veto that GWC was intended only as an introductory clause for most of the clauses in Section 8 which were delegations of specific powers.
Veto of federal public works bill
In other words, while Congress had problems interpreting GWC as Constitutional Convention delegates had intended for it to be understood, the Court likewise had problems interpreting Section 8's "necessary and proper" clause (1.8.18) in McCulloch, inadvertently turning that clause into the "convenience" clause imo.
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The Federal government has only the ‘ennumerated’ powers under our constitution.