Your post grammar seems pretty good-- there are some spelling errors and errors of register (formality)-- using abbreviated forms, for instance-- but otherwise, no big problems are evident. Keep reading the sorts of things you need to write. Google for 'business letters', 'writing style' (here's one: http://www.unc.edu/depts/wcweb/handouts/style.html ), etc. and note the styles and vocabularies.
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