39 C.F.R. § 121.1 : Postal Service — UNITED STATES POSTAL SERVICE — POST OFFICE SERVICES [DOMESTIC MAIL] — SERVICE STANDARDS FOR MARKET-DOMINANT MAIL PRODUCTS — First-Class Mail

(a) A same day service standard is applied to USPS Connect TM Local Mail pieces properly accepted at participating Destination Delivery Units before the day-zero Critical Entry Time (CET); a one day service standard is applied to USPS Connect TM Local Mail pieces accepted via carrier pick-up or properly accepted at participating Destination Delivery Units after the day-zero CET.
(b) A 1-day (overnight) service standard is applied to intra-Sectional Center Facility (SCF) domestic Presort First-Class Mail pieces properly accepted at the SCF before the day-zero Critical Entry Time (CET), except for mail between Puerto Rico and the U.S. Virgin Islands, and mail destined to American Samoa and the following 3-digit ZIP Code areas in Alaska (or designated portions thereof): 995 (5-digit ZIP Codes 99540 through 99599), 996, 997, 998, and 999.
(c) A 2-day service standard is applied to:
(1) Intra-SCF single-piece domestic First-Class Mail properly accepted before the day-zero CET if:
(i) The SCF is also the origin Processing & Distribution Center or Facility (P&DC/F), or
(ii) the combined drive time between the origin P&DC/F, destination Area Distribution Center (ADC), and destination SCF is 3 hours or less;
(2) inter-SCF domestic First-Class Mail pieces properly accepted before the day-zero CET if the combined drive time between the origin P&DC/F, destination ADC, and destination SCF is 3 hours or less;
(3) Presort First-Class Mail properly accepted before the day-zero CET with an origin and destination that are separately in Puerto Rico and the U.S. Virgin Islands; and
(4) intra-SCF Presort First-Class Mail properly accepted before the day-zero CET with an origin or destination that is in American Samoa or one of the following 3-digit ZIP Code areas in Alaska (or designated portions thereof): 995 (5-digit ZIP Codes 99540 through 99599), 996, 997, 998, and 999.
(d) A 3-day service standard is applied to domestic First-Class Mail pieces properly accepted before the day-zero CET, if the 1-day and 2-day service standards do not apply, the combined drive time between the origin P&DC/F, destination ADC, and destination SCF is 20 hours or less, and both the origin and the destination are within the contiguous 48 states.
(e) A 4-day service standard is applied to domestic First-Class Mail pieces properly accepted before the day-zero CET, if the 1-day, 2-day, and 3-day service standards do not apply, and:
(1) The combined drive time between the origin P&DC/F, destination ADC, and destination SCF is 41 hours or less, and both the origin and the destination are within the contiguous 48 states;
(2) The origin is in the contiguous 48 states, and the destination is in any of the following: The city of Anchorage, Alaska (5-digit ZIP Codes 99501 through 99539); the 968 3-digit ZIP Code area in Hawaii; or the 006, 007, or 009 3-digit ZIP Code areas in Puerto Rico;
(3) The origin is in the 006, 007, or 009 3-digit ZIP Code areas in Puerto Rico, and the destination is in the contiguous 48 states;
(4) The origin is in Hawaii, and the destination is in Guam, or vice versa;
(5) The origin is in Hawaii, and the destination is in American Samoa, or vice versa; or
(6) Both the origin and destination are within Alaska.
(f) A 5-day service standard is applied to all remaining domestic First-Class Mail pieces properly accepted before the day-zero CET.
(g) The service standard for Outbound Single-Piece First-Class Mail International TM; pieces properly accepted before the day-zero CET is equivalent to the service standard for domestic First-Class Mail pieces originating from the same 3-digit ZIP Code area and destined to the 3-digit ZIP Code area in which the designated International Service Center is located.
(h) The service standard for Inbound Letter Post pieces properly accepted before the day-zero CET is equivalent to the service standard for domestic First-Class Mail pieces destined to the same 3-digit ZIP Code area and originating from the 3-digit ZIP Code area in which the designated International Service Center is located.

39 C.F.R. §121.1

77 FR 31196 5/25/2012, as amended at 79 FR 4080, 1/24/2014; 79 FR 44701, 8/1/2014; 86 FR 43952, 10/1/2021; 87 FR 73469, 1/22/2023