I Am the Messenger by Markus Zusak

Ed Kennedy is an underage cab driver without much of a future.
He's pathetic at playing cards, hopelessly in love with his best friend, Audrey, and utterly devoted to his coffee-drinking dog, the Doorman. His life is one of peaceful routine and incompetence - until he inadvertently stops a bank robbery.
That's when the first ace arrives in the mail.
That's when Ed becomes The Messenger.
Chosen to care, he makes his way through town helping and hurting (where necessary) until only one question remains: who's behind Ed's mission?