EAST HAMPTON, N.Y. — Sometimes, Valerie Smith reminds herself that she is not a pollster, she is just selling cups — plastic drinking cups with the presidential candidates’ names. But she has been keeping track, just like a pollster, and hoping that her cup count is as reliable as it was in the last three presidential races.

The tally on the final weekend before Election Day — the number of cups sold in her shop here since just after the Super Tuesday primary contests in March, when she started counting in earnest — was 4,946 for Hillary Clinton, the Democratic nominee, and 3,388 for Donald J. Trump, her Republican rival.

If the cup count is wrong, Ms. Smith will join famous egg-on-their-face prediction makers like The Literary Digest, which had a perfect record for 20 years until it called the election of 1936 for Alfred M. Landon over President Franklin D. Roosevelt. Or The Chicago Tribune, which was so sure in 1948 that it published the infamous banner headline “Dewey Defeats Truman.”

Ms. Smith’s count is not the usual pre-election survey. It is based on a random sample — whoever walks into her store, the Monogram Shop, and buys the cups. But it is skewed toward the extremely wealthy. East Hampton’s average household income is $126,382 a year, almost two and a half times the national average. And the results can be clouded another way, when customers buy more than one cup. The cups are labeled with campaign logos she has copied when they are available, or designs she has created when they are not.