the cost of renting a po box the cost of a usps po box in your area depends on a number of factors including your location the size of the box and the rental periodin some areas a small box can be as little as $ 14 for a three month period in other areas the same box can be twice that amountas the boxes get larger the cost gets higherhe cost of renting a po box the cost of a usps po box in your area depends on a number of factors including your location the size of the box and the rental period