Section 8 of the Constitution states that Congress shall have Power to collect Taxes and provide for the general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States;
The Postal Accountability Enhancement Act forces the USPS to prepay money for the pensions and benefits of workers not even born 10 years down the road. This act is unique to the post office - no other governmental department is being forced to fund future pensions or benefits even though the USPS does not receive a single penny from the government - their funding comes from the sale of postage.
The Post Office has existed since the 1700s and is not going broke. The only reason this was passed was to bankrupt the Postal Service so that it could be privatized.