Dear Felix
Regretfully, unfortunate circumstances have caused us to write this letter to you.
We are the holder of a [check / note] to which you are an endorser; a copy of this [check / note] is enclosed. Since payment on this [check / note] is in default, we must demand that you, as endorser thereto, fully pay the outstanding debt due in the amount of $[AMOUNT].
Regards
Lydia