OOC:
To summarise the main points of the letter:
Jack needs you – or some other group of gunslingers – to get to the Tower, so he is trying to help you get there.
However he's also a deity of chaos and malice. So while he will help you, he will also throw malice and chaos in your path. If that stops or kills you that is annoying for him, will delay him, but he'll find other gunslingers...
He includes two cards in the package that describe the next challenge he has put in your path. If you get through/past this challenge, you will come to a door that will bring you closer to the Tower AND you will each get a gift to help you on your way i.e. your own personal card.
He also gives you a jack-in-a-box. This is a demon that will always, foul-mouthedly, tell you which direction the Tower lies in.
And finally the person who opened the package (Henry) got an extra "gift" i.e. his own personal demon.