In most USPS locations, you will not be able to pick your box number. Depending on the demand for boxes at that location, you may not even be given a choice - you will be advised which box is available in the size you need and you take what you can get. Otherwise, if more than one box is available, you may be allowed to select the post office box you desire.

Most commercial post office box services have a sufficient number of available boxes that prospective renters have a choice.

In all cases, however, renters are at the whim of the numbering system! The system must be easy for the employees who distribute the mail to the boxes.

No post office station is identical to another. Post office stations in less-populated areas are smaller than those in the central business district. The size of the station directly affects the number of post office boxes it can accommodate. More or less, there are a hundred PO boxes in a rural post office station. On the other hand, a CBD (commercial business delivery) post office station can have more than 100,000 PO boxes.

Considering the quantity of PO boxes in a station, it is important to work out an effective numbering scheme. Also, the number of the PO box must be unique to avoid a tricky situation.

Normally, post office stations in the same area are using cumulative numbering schemes. To give a better picture, the boxes in the Main Office are from 1 to 999 while the numbers 1000 to 1999 are for boxes in the West Office.

In United States, several cities include the ZIP code as part of the numbering scheme. For example, in Chicago, Illinois, post office boxes beginning with 11 are in the vicinity of the ZIP code 60611. Post office box numbers are also tied to the +4; i.e., using the above example, box 11265 would be ZIP code 60611-1265.

In the US, post office boxes are typically numbered from left to right, then top to bottom from the viewpoint of the postal worker. From the point of view of the box holder, the boxes are numbered from right to left.

Charges for renting a post office box depend on the size of the post office box and frequently the location, as well. Prices start as low as $10/month (depending on box size and location). Some USPS post offices have a lengthy waiting list for available post office boxes, which has given rise in urban and business areas to private post office box services. Private post office box service rates also vary according to demand.

In the United States, standard box sizes are:

• Small box 1: 3" x 5.5"
• Small box 2: 5" x 5.5"
• Medium size 3: 11" x 5.5"
• Large size 4: 11" x 11" (drawer)
• Large size 5: 22.5" x 12" (drawer)
• Depth of most post office boxes is 14.75"

The location also has an impact on the cost of the box. Central business district (CBD) PO boxes are in higher demand and command a higher price compared to rural PO boxes. The rates in renting a PO box in the United States used to be standardized. Now, the rental rate is divided into seven fee groups, which are based on the location of the post office station.

In the United States, there are customers who may be entitled to free post office box service.