Would it be "A Blacksmith Courted Me"? Some versions involve writing a letter, perhaps this was a variation on it?

https://mainlynorfolk.info/shirley.collins/songs/theblacksmith.html

A blacksmith courted me nine long months and better,
At first he won my heart wroted me a letter;
With his hammer in his hand, he strike both neat and clever,
If I was with my love I would live for ever.

“Don't talk of going abroad, fighting for strangers,
You stay at home young man, free from all danger;
You stay at home young man, with your lovely jewel,
And fold me in your arms, love and don't be cruel.”

“You promised to marry me when you first laid by me,
You promised you'd marry me love, you'd not deny me.”
“If I promised to marry you it was only to try you,
Go and fetch your witness here, love, I won' deny you.”

“ Oh witness have I none, save the sky above me,
And you' be rewarded well for the slighting of me.”
Her lips grew pale and white, it made her poor heart tremble,
To think she loved that one that proved deceitful.

“ Now when I had gold in store, you seemed to like me,
But now I'm low and poor, all you're going to dislike me;
You courted me for a while, just to receive me,
But now my heart you have won, Love, you're going to leave me.”

My true love's across the sea, with a bunch of posies.
My true love's across the sea, with his cheeks like roses.
I'm afraid the rising sun will spoil his beauty,
If I was with my love, I would do his duty.

Sad news have come to tell, sad news is carried,
Sad news have come to tell that my true love's married;
I wish him well to-do, although he ain't here to hear me,
I would never die for love, young girls, believe me.

Down on the ground she fell, syphing and crying
Throwing her arms abroad like one a-dying;
There is no belief in a man, if it's your own brother,
So young girls when you love one, you love one each other.