By default, rates from UPS and FedEx fail when someone inputs an address containing 'PO Box, P.O. Box, P.O Box'
For other rates and carriers, you need to add a rule within the rate that looks like:
"Hide rate when...Address...contains...'PO Box, P.O. Box, P.O Box, P O Box"
Edit any rate within the app
Scroll to the Rules section
Choose the Rule Type 'Hide rate when....'
Click 'Add Rule'
Change the first dropdown to 'Address'
Choose 'contains'
In the input field, put 'P.O. Box, P.O Box, PO Box, P O Box'
How to set a rate to only show when there is a PO Box address.
So if you have hidden a rate from showing for PO Boxes, maybe you want a special different rate to show instead. Commonly people will show a USPS Calculated Rate for PO Boxes, or some other custom rate.
Here is how to do that.
Add a rate that you want to have for PO Boxes.
In the Rule section of that rate choose the rule type 'Only show rate when...'
Change the first dropdown to 'Address'
Choose 'contains'
In the input field, put 'P.O. Box, P.O Box, PO Box, P O Box'