I need help with an offer letter!!

[Josh Longtin]

I am submitting an offer on my first flip property.  The property is vacant, and partially rehabbed.  My offer is 20% below asking.  I don't want to insult the seller, but the scope of work dictates a lesser offer in order to make my formula work.  I have been told that a personal letter accompanying the offer can go a long way.  Has anybody written one before and care to offer any tips?  I am having trouble with the proper verbiage in order to communicate my interest without looking like some lowballer.