Post Office Boxes

PO boxes or Post Office boxes are rentable, lockable boxes in PO stations. The Mexican Mafia and Sureños utilize them to receive monies owed to them. It's too risky to have inmates transfer money to their inmate trust funds because they can be seized and shut down at any time. The PO boxes are rented by somebody that's on the streets (either a wife or other family members.) When inmates purchase drugs or pay taxes, they are given the address to the PO box and then they contact one of their own family members on the streets. That family member then mails out a payment to the PO box and the family member of the Carnal or Sureño collects the payment. (Note: This is how most drug transactions and tax "kick-ups" take place. For small transactions inmate trust funds are still utilized.)

Your family members on the streets can be NPC'd, even though it's not preferred.

Examples of PO Box addresses (if you want to use one IG):

1201 BROAD ST E (Street & building number where the PO Box is located)
PO BOX 1001 (PO Box number itself)
FALLS CHURCH VA 22062-1001 (City, State, ZIP+4 Code)

A ZIP+4 Code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting. Basically it's just a ZIP code.

Worth a read again.