Before I even read the title or realized the point of the article, I noticed they used the word "envelops" instead of envelopes in the introduction... What irony. :)
Here are some more mistakes I found: Give the bonus only to people that [WHO] respond to the offer in your main piece.
Here's one I found that same day, in the PDF article: (www.pdfonline.com). PRF Online is an easy to use service to create PDFs.