Ok...I think maybe you made a mistake on this one, but here goes:
P.S. I Love You
Are both definitely in the "form" of a letter....however, I cannot think up another song that fits this mold. I know plenty of other songs which talk about writing a letter, but are not actually in the form of a letter....The closest thing I can think of is:
Which opens with a greeting similar to a letter, but there is no evidence of a letter being written.
I would really like to hear the 3rd song, because I am stumped.