Warning This Section Does Contain Spoilers
This letter from Heather tells us she deeply cares for him. It doesn't have a date but she mentions sending him a mixed tape to comfort him in his dark, cold, and lonely days.
She speaks of Allan, her boyfriend, who has wronged her. He has broken her trust and betrayed her love. She mentions paying him a visit when his parents are out and righting the wrong.
She laments that no one but JWJ "gets her" and that is her favorite thing about him. And the fact that he tries to get her. She wishes they could meet soon and mentions that she hopes he gets that bottled water soon.