"Congress is not empowered to tax for those purposes which are within the exclusive province of the States." --Justice John Marshall, Gibbons v. Ogden, 1824.
In other words, Congress cannot lay taxes for anything that it cannot justify under Section 8 of Article I of the Constitution, SS and Obamacare being glaring examples.
And the only reason that corrupt Congress has been able to establish constitutonally indefensible taxing and spending programs is the following imo. Voters abused their voting power when they repeatedly elected FDR in support of his constitutonally indefensible New Deal federal spending programs as a consequence of likely widespread ignorance of constitutional limits on Congress's powers to regulate domestic government services.
In fact, defense issues aside, one of very few federal government services that citizens should be unquestioningly paying taxes for is the postal service (1.8.7).
Visibility BUMP.
Okay, I'm probably picking a Constitutional nit, but the general government has NO authority to 'tax' the People because of 1.8.7. To charge for postage (a usage fee), but not to tax.
The scope was 'limited to the mere right of passage and preservation' according to Story's View from 1833.
§ 1140. But it is said, that it would be dangerous to allow any power in the Union to lay out and construct post-roads; for then the exercise of the power would supersede the state jurisdiction. This is an utter mistake. If congress should lay out and construct a post-road in a state, it would still be a road within the ordinary territorial jurisdiction of the state. The state could not, indeed, supercede, or obstruct, or discontinue it, or prevent the Union from repairing it, or the mails from travelling on it. But subject to these incidental rights, the right of territory and jurisdiction, civilly and criminally, would be complete and perfect in the state. The power of congress over the road would be limited to the mere right of passage and preservation. That of the state would be general, and embrace all other objects. Congress undoubtedly has power to purchase lands in a state for any public purposes, such as forts, arsenals, and dock-yards. So, they have a right to erect hospitals, custom-houses, and court-houses in a state. But no person ever imagined, that these places were thereby removed from the general jurisdiction of the state. On the contrary, they are universally understood for all other purposes, not inconsistent with the constitutional rights and uses of the Union, to be subject to state authority and rights.
Commentaries on the Constitution
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Nice catch on Gibbons v. Ogden, BTW. Most folks don't have the acumen to understand its importance.