That's a tough nut...symbolically it is hard to say thank you, when trying to come up with something I think in terms of face to face and when I think of genuine thanks, I picture an extended hand and a slight bow from the neck, or a specific look in someones face when you can tell they are thankful. For an icon that would be tough to make and make it fit.
All I can think of is an envelope with a thank you card something like this here (don't mind the crudity please..just scraped it together out of some stock and some quick PS edits.)
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