Letter (first weight step) and post card rates to the United States increased from 8 cents to 55 cents in the last quarter of the twentieth century. This post looks at the first weight step rate changes from 1972 to 2000.
A. Surface - Air Mail Period
8 cents Surface Rate : January 1, 1972 - February 29, 1976
The surface letter rate was 8 cents for the first ounce.

Calgary to Buffalo, May 3, 1974
8 cents surface letter rate for one ounce
10 cents Air Mail Rate : November 1, 1968 - February 29, 1976
The air mail rate was 10 cents for the first ounce. The 10 cent air mail rate had been introduced on November 1, 1968.

Red Deer to Port Chester, N.Y., April 28, 1975
10 cents air mail letter rate for the first ounce
B. All-Up Service
The surface/ air mail rate distinctions were abolished. All letter mail to the United States was forwarded the most expeditious way.
10 cent All-Up Service : March 1, 1976 - March 31, 1977
The rate for all letter mail to the United States was 10 cents.

Richmond, B.C., to Jackson, Miss., January 17, 1977
10 cents all-up letter rate

Aylmer West to Portland, Oregon, April 22 1976 (7:00 am cancellation)
8 cents affixed originally, thus shortpaid 2 cents
The letter was returned to the sender for additional postage:

2 cents added (Macdonald definitives) and cancelled at Aylmer West, August 22 1976 (10 p.m. cancellation)
12 cents: March 1, 1977 - March 31, 1978

Lachine to Milwaukee, October 11, 1977
12 cents letter rate
14 cents : April 1, 1978 - March 31, 1979

Kapuskasing to New York, July 31, 1978
14 cents letter rate for the first ounce
17 cents (up to 1 ounce) :
April 1, 1979 - June 30, 1979

Mississauga (L4W 1T0) to Sun City, Ca., May 1, 1979
17 cent letter rate for one ounce
17 cents (up to 30 grams) :
July 1, 1979 - December 31, 1981
Letter weights were now measured in grams.

Macamic to Holland, Mich., November 21, 1981
17 cents letter rate for 30 grams
35 cents : January 1, 1982 - January 14, 1983

Toronto (M4L 3T0) to Tamarac, Florida, September 17, 1982
35 cents letter rate for 30 grams
37 cents : January 15, 1983 - June 23 1985

Kazlo to Daly City, July 6, 1984
37 cents letter rate for 30 grams
39 cents : June 24, 1985 - March 31, 1987

Kingston, N.S., to Arlington, Va., December 4, 1986
39 cents letter rate
Forwarding label affixed
42 cents : April 1, 1987 - December 31, 1987

Meteghan River, N.S. to Mount Clemens, Mich., December 10, 1987
42 cents letter rate
43 cents : January 1, 1988 - December 31, 1988

Toronto to San Diego, August 21, 1988
(Change of Address Card)
43 cents letter/post card rate
44 cents : January 1, 1989 - December 31, 1989

Salmon Arm to Anderson, In., July 4 1989
44 cents letter rate
45 cents : January 1, 1990 - December 31, 1990

Scarborough to Clearwater, May 19, 1990
45 cents letter rate
46 cents : January 1, 1991 - December 31, 1991

Comox to Palmyra, N.Y., December 23, 1991
46 cents letter rate
48 cents : January 1, 1992 - December 31, 1992

Port Colborne to Buffalo, November 9, 1992
48 cents letter rate
49 cents : January 1, 1993 - February 28, 1994

Beauvallon, N.B., to Indianapolis, July 16, 1993
49 cents letter rate
50 cents : March 1 1994 - July 31 1995

Nanaimo to Longview, Wash., April 11, 1995
50 cents letter rate
52 Cents : August 1, 1995 - December 31, 1998

Toronto to Chicago, May 1, 1998
52 cents letter rate
55 cents : January 1999 - December 31 2000

Saint John to Chicago, December 1, 2000
55 cents letter rate to end the 20th Century