Selected Privileges and Courtesies Extended to Former Members of Congress [Updated September 15, 2020] [open pdf - 715KB]
From the Summary: "After Members of the House or Senate leave office, they are afforded certain courtesies and privileges. Some are derived from law and chamber rules, but others are courtesies that have been extended as a matter of custom. Some of these privileges and courtesies include the following:  access to the floor of the chamber in which a former Member served;  short-term franking privileges;  access to parking facilities and athletic or wellness facilities;  access to House or Senate administrative services and dining facilities; and  access to materials through the Congressional Research Service (CRS) and the Library of Congress."
CRS Report for Congress, R41121
Congressional Research Service: https://crsreports.congress.gov/