"Happy early Valentines Day everyone," she wrote on Twitter and Facebook. "I am sending this early because I am sending some of you and my family and friends this photo as a postcard."
The 19-year-old also reminded fans that they can send their own Valentine's Day card using the app, and offered up a special promo code for the occasion.
Selena teamed up with the app in November 2011. Postcard on the Run allows users to take a photo from their smartphone and send it as a high-quality printed postcard.
"I've always loved the old-fashioned touch of sending real postcards, but I never have time anymore with my busy travel schedule," she said in a press release. "Now it's easy with Postcard on the Run."
What do you think of Selena's valentine? We bet Justin Bieber likes it!