It would be incredibly helpful to, when creating shipping zones and rates, to have an oprtion of "exclude post boxes/military addresses".
We ship most of our international orders via UPS as they are considerably cheaper than USPS, but customers who have post boxes we still must ship USPS. This means the customer ends up undercharged for shipping and we have to eat that cost. Chasing customers down for additional funds is a headache and means the potential loss of the sale requiring a refund (which, if you process a lot of refunds, that effects your transaction rates and standing with your CC processor).
Much like a shipping tier which isn't applicable due to weight or region doesn't display to non-applicable orders, having certain shipping options not display for post boxes/military addresses would be incredibly helpful and reduce a lot of headaches for everyone involved. I don't want to exclude post boxes/military addresses entirely, just from certain shipping tiers.
I know this isn't a new issue- lots of old "outdated" threads which have been closed addressing this same topic. Odd that they're considered outdated and closed when this is still something that shops are having to deal with and there's still no solution.
I am Ryan from the Shopify Support team.
The best way to exclude a specific area from a Shipping Zone is to go into Settings > Shipping > Click Edit next to the United States. Once in this new page Click Edit next to United States (62 of 62 states). From here you can remove the two "post boxes/military addresses". Once done you will go back into the Shipping page and click Add Shipping Zone and then click Add County and type in United States. When this pops up you will be able to choose the two "post boxes/military addresses" you had removed. Now at this point you will have a new Shipping zone which is dedicated to this specific region of the United States and you will be able to set the rates you like for this zone.
Hope this helps :P
Sounds like you need a more solid solution here than Shopify can offer. PO Boxes can be intricate to handle as basically you are parsing the street address to work that out. We also see customers wanting to apply surcharges for certain price points and only certain products to outer US, by getting in a full rate management solution you can have something that will allow you to add rules as you grow.