The business of postage stamps and creation of new and varied stamps started simply but has become complex over the 150-plus years of operation. In recent years, some postage stamps and preposted postcards have not shown a value on the stamp. You can determine the postage value and use unused postage stamps, regardless of their age.
Refer to the Domestic Mail Manual provided by the U.S. Postal Service. The Quick Service Guide 604a shows pictures of nondenominated postage (see Resources).
Review the stamps that have no denomination shown. In 1978, the Postal Service started the ABC series. The A stamp was the eagle on an envelope with a 15-cent value. The B stamp was issued in 1981 with the same appearance as the A stamp, but with the letter B and a value of 18 cents. The B postcard issued at the same time did not have an initial but was otherwise the same eagle stamp, with a denomination of 12 cents. The C stamped envelope issue arrived in 1981, carried the same eagle in flight and a 20-cent value. The last time the eagle appeared on this postage was for the D stamp, issued in 1985 on an envelope at 22 cents. The earth stamp was E, issued in 1988 for 25 cents. F had two designs--a tulip and a flag--both issued in 1991 at 29 cents, and to accompany the F was a "makeup stamp" for 4 cents.
G carried the American flag design, and had three formats. There was a 20-cent postcard price, a 25-cent presorted first-class price and a 32-cent for U.S. addresses only. The G makeup stamp has a dove and is valued at 3 cents. Uncle Sam's hat is on the H stamp issued in 1998 for 33 cents. There was also an H makeup stamp of 1 cent, with a rooster weather vane.
Look for semi-postal stamps. The best-known stamp in the semi-postal series is the breast cancer stamp, "Fund the Fight, Find the Cure," issued in 1998 at 40 cents, with 32 cents for postage and eight cents for the breast-cancer cause. In 2002, the Postal Service issued "Heroes of 2001" with a postage value of 37 cents and a purchase price of 45 cents. "Stop Family Violence" had a stamp value of 37 cents and a price of 45 cents. These stamps help fund the cause on the stamp. The breast cancer stamp is available at 55 cents in 2010, with 44 cents in postage value.
Find forever stamps with the Liberty Bell. Purchase these postage stamps at one value, and use them as first-class postage on a 1-oz. letter at any time. They do not require a "makeup stamp" or extra cost. These stamps first issued in 2007 continue to be available at the current first-class postage rate.
Locate other Postal Service stamps with no denomination marked on the face. There are others shown on the Quick Service Guide that are no longer available at the post office, but are usable if you have some. These include some Christmas issues, the antique toys in four versions issued in 2002 at 37 cents and a 41-cent American flag issued in 2007.