In your first example, a pause there sounds a little awkward -- but putting a comma there also looks awkward. To my ear it sounds sort of antiquated. In the s

In your first example, a pause there sounds a little awkward -- but putting a comma there also looks awkward. To my ear it sounds sort of antiquated. In the second example, adding a pause at that location sounds quite natural. It changes the meaning slightly, adding gravity to the statement, making the thank-you sound more heartfelt. [I'm a native English speaker, and the commas-as-pauses way of thinking has always worked very well for me...]