The optional DynamicZip Zip+4 Option when used with the Zip+4 File provides a way to positively identify the Tax Location county. ZIP Codes can cross county lines. The ZIP Code with the +4 extension does not cross county lines, and can therefore be used to determine the county based on the +4 extension.
NOTE: If your addresses don't include the +4 extension, the functionality added by this option is of no value. DynamicZip does not calculate the +4 extension for you.
The caveat of this option is that the data source (the Rate & Boundary Database optional Plus4.txt file) does not identify the city when more than one choice exists for the ZIP Code. Most address validation software that is used to obtain the Zip+4 extension based on the street address returns the USPS Preferred City Name. As a result, if you enable the Select Default option, DynamicZip will return the tax schedule for the USPS City and the ZIP+4 County. While this selection is for the correct state and county, it may be for an incorrect city. For optimum accuracy in tax schedule selection, we recommend that you do not enable the Select Default option when using the Zip+4 Option This way when a ZIP Code is selected that is used by more than one city, the user can be prompted to make the proper choice manually.
Prior to Build 72 there was an additional caveat that was related to using USPS Preferred City Name. As of Build 72 this is a ZipAddressing option that does not affect the Rate & Boundary Import process. Prior to this build, this option did affect the Rate & Boundary Import process in that it would replace the actual city with the USPS city for purposes of building and maintaining the tax schedule. This resulted in making the manual selection of the actual Tax Location city impossible in cases where the actual city was not the USPS city.
This situation exists in almost every state. The 10 states most affected are (in descending order): TX, IL, IA, MO, OH, GA, IN, MN, KS, and PA. With the ZIP+4 feature operational, DynamicZip can pinpoint the exact county for any ZIP Code for which you enter the +4 extension. At the time these calculations were made, 10.5% of the ZIP Codes cross county lines where at least one of the counties has a county sales tax.