The American flag is the reason you need to deliver this package. The flag is inside the package, and you need to deliver it to the president. He lost his old flag, so now it’s up to you to deliver this new one to him. What more of a reason do you need?

To deliver this flag is something you just have to do (とど). Need to do. Want to do. There is no one else to do it. This is something only you are equipped to do.