Hello @Irena  One simple way using apps(shipping customization) to conditionally hide shipping rates. No need CCS. 

Hello @WoooSaaaW  Yes, apps available on the Shopify to customize shipping, means conditionally you can hide/delete shipping rates if address contains PO box. (in your case you just need to select all shipping rates and condition contains address: po...

Hello @JordyPakes , Can you please share your Shopify store name?

Apps available on the market now to hide certain shipping if customer has no tag. it means you have to create both rates from Shopify.  EX: standard and premiumand you can hide shipping rates by customer tag vice-versa.