"Hardship" or "Medical Problems" is defined as an illness or handicap which would present a physical challenge for an individual to retrieve mail.
To request door delivery, you need to write a letter requesting this change and attach a statement from a Doctor. The doctor's statement should indicate you are unable to collect your mail from a curb or centralized mailbox. Both your letter and the doctor's statement must be sent to the Post Office™ that delivers your mail for approval or denial. Final determination on whether or not door delivery will be granted will be made by the Post Office.
Request letters should be addressed to:
POSTMASTER
UNITED STATES POSTAL SERVICE
<YOUR CITY, STATE AND ZIP CODE>
Note: Any approval will be temporary (further information can be obtained from the local Post Office).
The foregoing is from https://faq.usps.com/s/article/If-I-have-Hardship-or-Medical-Problems-how-do-I-request-Door-Delivery
The door delivery exception for the physically handicapped has been codified by a 1998 law: "Mail Delivery to Physically Handicapped Act."