American Idol winners are competing once again. This time, on a tiny postage stamp. The last of the Idol Gives Back limited edition photo stamps featuring past Idol Winners such as Underwood, Fantasia and Ruben Studdard (who bought a bunch for his family and friends) was issued last week with newly inducted AI winner David Cook joining the mix. The most popular stamp will be Underwood's, predicts Joseph Savarese of The American Stamp Dealer Association. "Because she's one of the most popular stars of the entertainment world today." But figuring out which stamp will be more valuable is another story, says Savarese. In 1992, it seemed the whole world wanted to buy the 29-cent Elvis Presley stamp. "Today this stamp in the secondary market sells for less than a dollar," he says. Whether or not her stamp is the most valuable doesn't seem to matter to Underwood. "It's pretty amazing to think that my face is actually on a U.S. postage stamp," the Grammy winner told PEOPLE exclusively. "Usually it's Presidents who get their pictures on stamps!" The stamps can be purchased at Photostamps.com and proceeds go to the Idol Gives Back Foundation. -from PEOPLE.com