https://www.irs.gov/pub/irs-pdf/i1040gi.pdf            see page 15

you may use a PO Box only if USPS does not deliver to your home

I didnt realize that, I have LOTS of clients that use PO Boxes because the mail at their home isnt secure and they dont want important mail going there.

I didn't know that either.  I also have a few that use a PO box.

Your client can ask if their PO station offers the Street Addressing Service:

Street Addressing Service for your PO Box

With this service, you will have the option of using the street address of this Post Office location for your mailing address in addition to your PO Box number. For example, if a Post Office is located on 500 Main Street and you have PO Box 59, your mail would be addressed 500 Main Street #59. You will also be able to use the street address to receive most packages and deliveries through private carriers, such as UPS and FedEx. Both Street Addressed mail and PO Box addressed mail can be delivered to your PO Box.