Each carrier method you add needs a Ship From Postal Code in order to be saved, and for that method to work.
Sometimes this is also referred to as an Origin Zip Code.
When we send information to a carrier's API we need to tell them where the product is shipping from and where it is shipping to.
The app will tell the carrier where the product is shipping to based on the customer's address.
The Ship From Postal Code is used to tell the carrier where the product is shipping from.
The first method's Ship From Postal Code will be used as default for other methods added to that zone.
Most of the time, the same ship from postal code is used for all methods within a specific shipping zone for the same group of products.
So after you create your first carrier method and enter a ship from postal code, and then you add a second carrier method, we will clone the ship from postal code from the first method into each subsequent method.
There are some cases, potentially, where you'd want to specify different ship from postal codes. Let's say you have two warehouses. One of them only ships via USPS, and the other ships with multiple carriers. You could set different Ship From Postal Codes for each shipping method.