Answer:
The postal clause gave the power to the congress To establish and manage Post Offices and post Roads. In order to give this power, the federal government has to provide the congress with proper funding.
In today's day and age, sending letters through post office is extremely inefficient. You can achieve the same result by using Email which cost you nothing and require much lesser effort.
As for delivery that contain physical goods, Delivery companies that is operated by the private sector tend to do it much more efficiently compared to the government. They can deliver the goods with a much faster time and less rate of damage goods.
Because of this, that Funding can be allocated to other government programs instead. (maybe to health care or public infrastructure)
