I think the pattern on the dress and background was unusual enough to make it into an envelope, but also hard to choose a stamp for it. This is one made of thin, glossy magazine paper - not the most durable.
Stamp chosen to somewhat match the pink (pity there isn't a pink 'waves of color' stamp - although maybe the $1 waves stamp would have been better...)
Lettering colored to match the stamp. I did like how the blue looks with this font:
And the back:
I didn't realize how obvious the underwear was until after it was stamped,addressed and sealed to send! Ah, well, such is life. Don't worry, I don't think there'll be an 'underwear' subseries.