The point is that despite constitutional authority it is not mandatory.
The ‘post roads’ authority was used for pre-interstate highway fed funding. Ike used ‘common defense’ authority for the Interstates, too big a deal for ‘post roads’.
The "national defense" excuse had to do with justifying the entire program but the specific clause used for the tax and subsequent legislative funding will be "post roads".
They build bridges and airports under that clause.
When it comes to "roads" not all roads are "post roads" ~ unless the state/local government declares it to be one and there's no federal funding.