Even if you are prepared to work hard for long hours, other people may not be - so be ready to hear them complain at times.
Because everything at the Post Office is based on seniority, the relationship between the Post Office's regular full-time employees, their "360s", who are not considered regular employees, but are on a yearly contract, and the people who are brought on for temporary positions, such as over the holidays, can be somewhat confusing at first. It can also be somewhat contentious when a manager's directions for a full-time employee or a 360 are not properly understood. If you have any problems or confusion related to who has what rights based on seniority, ask a manager and they will clarify.
If you are hired for a temporary position, it does not give you a guarantee of receiving a permanent position, so keep in mind that this is not likely to become a long-term vocational option.
If you are hired as a 360 (on a yearly contract) after you have worked a temporary position, expect to work very long hours and receive very little time off of work throughout most of the year. You will be told by other employees that you cannot, for any reason, miss a single day of work, or consistently clock in or out late/early by more than a minute or you will be fired and replaced.