Obvious option would be the law firm handling your business license. But can we also take a minute to appreciate the absurdity of a PO box ever being deemed unacceptable? It literally exists for this exact purpose, and there are any number of "PO box except not a PO box" schemes out there due to this issue. It ought to be illegal to treat PO boxes differently IMO.

Mainly they want an address if they need to serve legal notice to you. You can't deliver that to a PO box, it has to be handed to someone at a physical address.