Postal codes are the numbers that appear when you open your main GTA map using Escape.
You can set a postal by typing the chat command:
/postal ### where the
### is the postal code number.
/postal 871 to set a route to postal 871.
The postal waypoint will appear separate from your purple waypoint you place using the big map, allowing you to have two waypoints set.
TIP: You can type
/postal to clear any actively set waypoint.