Dear Charnchai,
If gives me much pleasure to receive your kind letter of congratulations on my promotion. It is correct for you to say my promotion is considered as the result of my ability and diligence in work.
I am proud of you, too.
Although you do not get your promotion on this occasion, yet you will, I think, get it in the very near future.
I avail myself of the opportunity to invite you to have dinner with me at my residence at 7.00 p.m. on the tenth of this month. A few friends will join me 011 that day. I hope you will accept my invitation.
Sincerely yours.
Sermsak