expression
this is just a short note; I'm writing to tell you
A formal, old-fashioned opening phrase used at the beginning of a brief letter, typically by men. It signals that the writer is getting straight to the point. Often followed by the main message without lengthy greetings.
一筆啓上、先日は大変お世話になりました。
I'm writing to tell you, thank you very much for your help the other day.
一筆啓上、このたびの件についてご連絡申し上げます。
This will just be a short note to inform you about the matter at hand.