Answer:
Dear Sally,
I would like to inform you that I have recieved your invitation to your birthday party. I must say that I cannot come to this event due to unfortunate events of me not wanting to come to it, though I would like to give you a little something. Happy Birthday, Sally. Unfortunately, that is all I can give you. Sorry for wasting your time, money, and supplies. Good day, Ma'am.
Sincerely, [Your name].
Explanation:
I dunno, you can change it up if you want to. It's just a little sum sum I wipped up :>
