英语作文朋友邀请参加生日聚会因有事不能去怎样写回信?

Dear **,
Iam terribly sorry. I can't attend your party. An unexpected visitor comes, and I have to accompany him for the rest of the day. So I guess I can't get to the party. Would you please acccept my apolgy. The present will arrives on time!(I have mailed it.)Happy birthday!
Yours,
*****

Dear XXXI am very sorry that I could not take part in your birthday party.Because I have to ...Sorry again.I really hope you can understand.Anyway,enjoy your birthday party.Best Regards