I've never used a P.O. Box before and I assumed that larger boxes would be stored inside the local post office branch and a slip put in the P.O. Box announcing its' arrival. When the P.O. Box owner got the slip, they just walked over to the counter to retrieve the larger box?
More and more people seem to be using the P.O. Boxes to avoid leaving packages on their front porches,especially in higher theft areas.