From the USPS website:
Employee Tipping and Gift-Receiving Policy
All postal employees, including carriers, must comply with the Standards of Ethical Conduct for Employees of the Exec*utive Branch. Under these federal regulations, carriers are permitted to accept a gift worth $20 or less from a customer per occasion, such as Christmas. However, cash and cash equivalents, such as checks or gift cards that can be exchanged for cash, must never be accepted in any amount. Furthermore, no employee may accept more than $50 worth of gifts from any one customer in any one calendar year period.
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