There is also another type of postal code, the ZIP + 4 code. According to unitedstateszipcodes.org, the ZIP + 4 was established in 1983 and identifies an even more specific location within a ZIP code.

The 54915 ZIP code can indicate Calumet and Outagamie counties, which have a 0.5% sales tax, but also Winnebago County, which doesn't have the tax.