USPS doesn't draw a boundary, it draws a route
The official unit of geography USPS uses is the delivery route, the path one carrier walks or drives to deliver mail. A ZIP code is simply a label that groups together a set of routes managed by one post office.
Because routes change with new construction, road closures, density shifts, and operational reorganization, ZIP code coverage is dynamic. A single ZIP can shrink, expand, gain a new sub-area, or be split into two ZIPs without any change in city or county boundaries.
Why USPS doesn't publish official ZIP polygons
USPS does not publish official ZIP polygon files because the underlying object (a set of routes) is not a polygon. Any shape you draw around a ZIP is an interpretation, and USPS does not want to certify interpretations it cannot operationally guarantee.
USPS does publish address-level data through its Address Information System and licensed AIS products to commercial subscribers, but these are address tables, not boundary files.
How the Census Bureau approximates ZIP boundaries with ZCTAs
Because so many users (government, business, researchers) need ZIP-level data, the Census Bureau built the ZIP Code Tabulation Area, or ZCTA, in 2000.
For every census block, the Bureau identifies the most common ZIP code in the addresses inside that block. Adjacent blocks that share a most-common ZIP get grouped into a ZCTA, which inherits that ZIP code as its label.
The result is the polygon you typically see on online ZIP maps. It is close to the real USPS coverage but not identical, because individual addresses within a block can belong to a neighboring ZIP.
Where commercial ZIP boundary maps actually come from
Almost every commercial ZIP boundary product (from GIS data vendors, mapping APIs, and consumer ZIP lookup sites) is derived from one of three sources: the Census Bureau's TIGER/Line ZCTA shapefiles, USPS licensed AIS products converted to polygons by the vendor, or geocoded address aggregation against a base parcel layer.
ZCTA-derived polygons are the most common because they are free, official, and updated decennially. Vendor-derived polygons can be more current but vary in quality.
ZIP codes can cross cities, counties, and state lines
Because ZIP codes follow routes, they do not respect political boundaries. A small number of ZIPs span state lines, where one post office serves addresses on both sides of the border. A larger number cross county lines. Almost every dense metropolitan ZIP spans multiple cities or census-designated places.
This is why you can have a Tampa ZIP that actually contains addresses in three different cities, or a Texas ZIP whose post office is just over the Oklahoma line. Census ACS data accounts for this through the ZCTA shape; commercial city-name databases sometimes do not, which is why you may see your address tagged with a city other than the one where you actually live.
How often ZIP boundaries change
USPS makes small ZIP changes constantly: adding addresses to existing routes, retiring obsolete codes, issuing new codes for large new developments. Major changes such as splitting a ZIP code or merging two are rarer and are usually communicated in advance to affected residents and businesses.
ZCTAs only update on the decennial census cycle (and to a lesser extent in annual ACS files), so the boundary you see on a ZIP map can lag the operational USPS boundary by months or even years.