Strictly speaking, U.S. Postal Service ZIP Codes are not geographical, but, rather, administrative constructs. They are actually a set of routes carriers take to deliver mail rather than a predefined geographic area with fixed borders. U.S. Postal Service ZIP Codes change frequently, are abstract in nature, and can be difficult to visualize. Therefore, it is quite difficult to get "true" ZIP Code boundary data, which is why the Census Bureau came up with ZCTAs in the first place.