Hi Guest, I would just use Ambassador Jones or whatever his or her name is, for both the salutation and body of the letter. I understand "Your Excellency" is sometimes used for ambassadors. It sounds too regal for my tastes. For example,

Hello. I'm writing from Japan. I have to send an invitation letter to some Ambassadors to Japan. I know I have to use "Your Excellency" or "Excellency" as Salutation, but should I repeat Excellency all the time in my main text in the letter?