Thanks for contacting ShippingEasy. Our application does not limit the option to select PO Box for other carriers, however, as mentioned in our post above, you can set rules to automate having those address types set to USPS.
The most general rule would be to create a saved selection that is just set to USPS, without a carrier service, or package type selected. This way you could switch the service to First Class, or Priority etc, as needed.
You can then add that saved selection to a rule.
The rule would simply say
Destination type > is equal to > PO Box
Set saved carrier selection > USPS PO Box (or whatever you decide to name that saved selection mentioned above)
This should make it so that your PO Box orders are always defaulted to USPS.
I hope this helps!