The 9-digit ZIP code dataset is available from the United States Postal Service (USPS) website, but it is not free. However, there are some open data alternatives you can explore to obtain the 9-digit ZIP code dataset for New York and Massachusetts.

US Census Bureau: The US Census Bureau provides a free TIGER/Line Shapefile dataset that includes 5-digit ZIP codes and 9-digit ZIP code tabulation areas (ZCTAs) for the entire United States, including New York and Massachusetts. The ZCTAs are approximate area representations of ZIP codes and include the 4-digit courier codes. You can download the data from the Census Bureau's website.

OpenAddresses: OpenAddresses is a platform that provides open data on address information for various countries, including the United States. The data includes 5-digit ZIP codes and, in some cases, the 4-digit courier codes. The data is available for download in various formats, including CSV and Shapefile.

OpenStreetMap: OpenStreetMap is a collaborative platform that provides open data on geographic features worldwide. The data includes 5-digit ZIP codes and, in some cases, the 4-digit courier codes. You can download the data in various formats, including Shapefile and CSV, from the OpenStreetMap website.

Data.gov: Data.gov is a portal that provides access to various open datasets from different US government agencies. You can search for datasets related to ZIP codes and geographic information for New York and Massachusetts on the website.

These open data sources may not provide the exact 9-digit ZIP code dataset you are looking for, but they can provide a good starting point and may include the 4-digit courier codes in some cases.

There are a lot of misconceptions regarding zip codes, especially down at the Zip+4 level. Simplistically, they are not 'areas', they are actually nets and sub-nets of a network - Excellent coverage of the issues at https://towardsdatascience.com/stop-using-zip-codes-for-geospatial-analysis-ceacb6e80c38

"The USPS ZIP Codes identify the individual post office or metropolitan area delivery station associated with mailing addresses. USPS ZIP Codes are not areal features but a collection of mail delivery routes.

Here we find our first problem with ZIP Codes, that they do not represent an actual area on a map, but rather a collection of routes that help postal workers effectively deliver mail. They aren’t designed to measure sociodemographic trends, as a business would generally want to do."

Because the codes at that fine level of resolution are practical logistics tool, they change on an almost daily basis according to the demands of the USPS and their customers, which is why the vendors charge so much, constant revisions are expensive,along with other value adds like crosswalk tables to navigate the changes as they occur.

Thilina D. Surasinghe

You can try checking the United States Census Bureau's website, which provides shapefiles for various geographic areas, including ZIP code tabulation areas (ZCTAs). ZCTAs are not the same as USPS ZIP codes, but they are approximate representations of USPS ZIP codes and include the 4-digit add-on codes. You can download the shapefiles from the TIGER/Line Shapefiles page on the Census Bureau's website.

Another option is to check with state and local government agencies in New York and Massachusetts to see if they have any GIS data available for ZIP codes with the 4-digit add-on codes. They may have more detailed or accurate data than the Census Bureau. You can also try searching for data on websites such as Data.gov, which provides a collection of open data from various government agencies.

If you are unable to find the data you need through these sources, you may want to consider contacting a local GIS professional or academic institution to see if they can assist you in acquiring the data you need.