Have the customers come to the PO to get their mail like their Great Grandparents did. The carrier craft might not like it though.
At 300 sq ft per parking space, and a peak period of half the customers between 3 to 6 PM, you would end up walking about 1/4 mile to get to your mailbox.
There are a variety of ways to compute this out, but it's an immense parking lot surrounding a building with lots of post office boxes.
BTW, when carrier delivery was established during the Civil War it was done to eliminate the very long lines of people showing up to check General Delivery.