Well for those in the States UPS Store gives you a physical address versus PO Box. I called them directly and they said it would be 25 per month for a mailbox. So I guess basically whatever location you choose that would be your street address then you mail number. Maybe what you can try is where ever you have you PO Box setup give them that location address and put your box number as the Apt/Suite Number.
Like for example a Post Office in Houston, Texas address is
401 Franklin St
Houston, TX 77201
So for example I would just put
401 Franklin St [BOX NUMBER]
Houston, TX 77201
And if something do get mailed to you I am pretty sure they would figure out it is suppose to go to a PO Box at that location.
So you can give the information to Google like that and it should suffice.