What do postcards symbolize within Chapter 12?

During a conversation between Amal and Adam, Adam confides that while he and his stepmother Charlene get along, he rarely sees his biological mother. Amal is shocked to learn that the only time Adam ever hears from his mother is when she sends him a postcard every year on his birthday. He adds that his mother has a new family and lives in Holland. The postcards in this scene, then, represent both physical and emotional distance between mother and son.