Dear XXX:
It is my great hornor to get your invitation to the 10th anniversary party of your company.I am sure you will have a great moment with your colleagues and your clients.Therefore,I should say, "Congradulations!" I am so proud of the development of your company and happy with you all guys who used to give me great help in the business.But it's a pity that I will have a business trip to the other city.I am afraid that I can't attend the party.I am so sorry for that.But I will send my present there before-hand to show my thanks and congradulations.
Best Wishes!
Sincerely Yours,XXX