Ignoring for a second the potentially sketchy offer, I have to say, this mode of marketing is absolutely brilliant.
What did it cost? $6.00 for a stamp, $6.00 for 500 Post-Its and a few minutes of manual labor. By strategically placing the Post-Its you have your viewers nearly undivided attention (the gas pump was perfect for this). Whoever left this Post-It appeared to do so sparingly, as the other pumps were missing notes. I think a little bit of subtly helps, too. Finally, the medium is portable, so that an interested party can simply take the note with them.
I love it.