What is a "Banker's Dozen" in the context of renting a PO Box?

A "Banker's Dozen" generally refers to a promotional offer where a customer pays for a certain quantity but receives an additional unit for free. In the context of renting a PO Box, this means that when you rent a box for 12 months, you are actually getting an additional month at no extra charge, resulting in 13 months of service while only paying for 12. This concept is frequently used in various retail and service industries to incentivize customers and encourage longer commitments, making it an appealing option for those needing a PO Box for an extended period.

The other options do not accurately capture the essence of the term. Renting for 12 months with the 13th month free conveys the same idea, but the wording doesn't align with the commonly accepted definition of a "Banker's Dozen." Buying money orders or adding an extra key for another person would not relate to the concept of receiving additional service time when paying for a set duration.