The latest update has added stickers to the game. These stickers can be attached to gifts as you send them to your friends. Stickers are completely aesthetic, adding no benefits to the gift they are sent with.
The game states that you can receive stickers from gifts or purchase them in the shop. However, I have not seen any available for purchase in the shop at this time. According to Serebii, there are currently 8 different stickers available; featuring Pikachu, Raichu, Ditto, Squirtle, Charmander, and Bulbasaur.