Earlier in the film, we see that Norm's working on a painting of some mallard ducks as part of a competition to get a picture on a stamp. He doesn't win the prize he wanted, however: he only gets his painting on the 3-cent stamp.
Norm tells Marge at the end of the movie, as they watch TV in bed: "Hautman's blue-winged teal got the twenty-nine cent. People don't much use the three-cent." But Marge is genuinely encouraging and proud of him:
MARGE: Oh, for Pete's sake, of course they do! Whenever they raise the postage, people need the little stamps.
Like Herman Melville says, they're both lowering "their conceit of attainable felicity." They're trying to be happy with what they have instead of searching for something really extraordinary. Apparently, the Gundersons are succeeding; they seem happy, awaiting the birth of their first child. One website about living the simple life thought that the phrase "three-cent stamp" would be their new shorthand for "be happy with what you've got."
P.S. Get this: on April 3, 1988—a few months after the movie takes place—the U.S. government raised the price of postage by exactly three cents (source). Marge is absolutely right, and Norm's three-cent stamp will soon be in much more demand than the design that ostensibly beat his.