USPS does have a contract.Because of his principles he is unable to work on Sunday.
Most jobs don't have a signed contract.
2021-2024 APWU/USPS Collective Bargaining Agreement | American Postal Workers Union
2021-2024 APWU/USPS Collective Bargaining Agreement The official and final version of the 2021-2024 Collective Bargaining Agreement. Changes agreed to by the parties, from the previous CBA, are in bold type.
apwu.org