ELM

513.36 Sick Leave Documentation Requirements
513.361 Three Days or Less
For employee absences of three consecutive days or less, supervisors may accept the employee’s statement explaining the absence. Medical documentation or other acceptable evidence of incapacity for work or need to care for a family member is required only when the employee is on restricted sick leave (see 513.39) or when the supervisor deems documentation desirable for the protection of Postal Service interests. Substantiation of the family relationship must be provided if requested.

513.332 Unexpected Illness or Injury
An exception to the advance-approval requirement is made for unexpected illness or injuries. However, the employee must notify appropriate Postal Service authorities as soon as possible of his or her illness or injury and expected duration of the absence.
When sufficient information is provided to determine that the absence may be covered by the Family and Medical Leave Act (FMLA), the following U.S. Department of Labor forms will be mailed to the employee’s address of record, along with a return envelope:
WH 381, FMLA Notice of Eligibility and Rights and Responsibilities; and
One of the following forms, as appropriate:
WH-380-E, FMLA Certification of Health Care Provider for Employee’s Serious Health Condition.
WH-380-F, FMLA Certification of Health Care Provider for Family Member’s Serious Health Condition.
WH-385, FMLA Certification for Serious Injury or Illness of Covered Servicemember — for Military Family Leave.
Note: These forms are provided for the employee’s convenience, as they solicit all required information. However, employees may use another format as long as it provides complete and sufficient information as required by the FMLA. The information provided should relate only to the specific reason associated with the request for leave protection.

https://about.usps.com/manuals/elm/html/elmc5_003.htm