There could be thousands of different types of reply letter,which fully depends on what the previous letter stated.
It can always starts as:
Dear Mr Smith,
I am pleased to receive your invitation.Thank you very much for your kindness.It would be my pleasure to attend your wedding ceremony on th 6th of October.As you suggested I would arrive at Palace Hotel at 6:30pm.Please pass my greetings to the beautiful bride and your whole family.
I am looking forward to seeing you all soon.
your
AQ