You write me love letters with your father's pen
If he knew the freaky, freaky things that you write with it
She is probably not literally writing letters with her father’s pen, but is texting him freaky messages (her father paying the phone bills).
This shows she is either not old enough to have a kid, or doesn’t have the financial independence required (or both)
To help improve the meaning of these lyrics, visit “LoveCrimes” by Frank Ocean Lyrics and leave a comment on the lyrics box