The USPS makes regular deliveries to PO Boxes and street addresses all around the US. From a financial aspect, the USPS actually prefers delivering to PO Boxes over normal street addresses. As you can very well guess, it costs less for the USPS to make a delivery to a PO Box than to your home or street address because every letter goes through the post office, and there is no need for transporting the letters to the street address if it is addressed to a PO Box. All the USPS people have to do is deposit the postal mail in the PO Boxes, which is the same place where all the letters, including the ones addressed to the same PO Box, are sorted. So it is the same equivalence of us putting something in our home from one place to another.