Once you reserve a PO Box at the Post Office location, link it to your USPS.com account. You can manage it online instead of the Post Office location. Link a PO Box to an USPS.com Account
Quick Answer. Cashier's checks themselves do not expire as personal checks do. However, the funds behind the check are subject to state escheat laws.
If the check is more than 6 months old or has a void date, ask for a replacement. Your bank may still honor an expired check, and the state may have a record of unclaimed funds for your check. Banks are not required to cash checks more than 6 months after the issuance date.