What's the difference between 보낼 and 보내줄
Many times 주다 is combined with other verbs to make a "compound verb." 주다 means "to give," but it also means "to do something for someone else's benefit."
편지를 아빠에게 순서달라고 되는데요
아들은 아빠에게 편지를 보내줄 거예요
Why not 아버지한테?