If you're going to the trouble of composing a well-thought out letter on beautiful stationary, why besmirch the whole effect with ugly, flag-emblazoned postage? Thanks to the wonders of the internet, you can upload your own pictures or designs to create custom postage. (I chose some pretty art from an Etsy designer.)
Stamp letters with your very own pictures. Great for cards, invitations. Or even put your company logo on one.