Zones are a tool used by USPS to measure the actual distance a package must travel. Zones range from Zone 1 (local) to Zone 9 (very far away). A zone is determined by the "ship from" zip code as it relates to the "ship to" zip code. The farther a package must travel, the more zones it crosses, and the most it costs to ship it.
Priority Mail Express and Priority Mail are zoned services. If a weight- or dimensional-based service is selected, then the zone will factor into the overall cost of the service. Flat Rate package types are exempt from zones.
Endicia provides a dynamic map illustrating zones across the country based on your zip code. Go to Endicia's zone map.
USPS provides a detailed breakdown of zones based on specific sets of zip codes. Go to USPS' look up tool.
USPS Priority vs USPS Priority Express: What's the difference?
What is Priority Mail Regional Rate shipping?
Best of Rules: Assign USPS Regional Rate Box to shipments with specific zones
What Canadian zone will be used for my Priority Mail International rate?
ShippingEasy's USPS Rate Tables