First of all, in your title, "advice" is always singular. " Asides from that, you have a good letter here. Here are a few errors I found.
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KirakiraI have to send this letter to an important customer. Please have a look at it and tell me if there are errors that I should correct. Thank you
"Dear Mr....
We are sorry for not having sent you the result of survey no 3. We did not receivereceivedy