Her boyfriend of five months, Eric Johnson, proposed to Simpson with a three-stone ring that features a central ruby flanked by two diamonds.
"She was in a really good mood," an eyewitness at the store tells PEOPLE. "She was very friendly and sweet. She signed autographs and seemed to have a great time."
Only a few weeks ago, the singer, 30, gushed about her beau, 31, to PEOPLE – but didn't want to "jinx" their relationship.
"He's the one for me right now!" she said. "I'm very happy! I'm in a great place, and if right now could last forever, I'd take it."
With reporting by ELIZABETH LEONARD and K.C. BAKER