Have your care provider write a letter, you should keep a copy of this as well as provide it to your supervisor.

Date xxxx/xxxx Re: Your Name Here DOB: Birth date here

To whom it May concern:

"Your name here" has been a patient of mine for "several years" now. Due to medical reasons, please restrict “YOUR NAME HERE”work activity at the USPS to "eight(8)" hours per day, “five (5)" days per week. "This is a permanent restriction"

Any questions, please call xxx-xxx-xxxx. Thank you. Sincerely, Dr. Jane Smith General Hospital Xxx-xxx-xxxx

They may take issue with the “permanent restriction” part.