The manufacturer of these envelopes claims they raise response by as much as 50%! We have not had an opportunity to test that number but we have had very positive feedback from clients who have experienced great results.
The key is to design a stunning mail piece with attractive cover art. We then insert the document into glossy transparent envelope. It makes for a very attractive mail piece that increases its chances of being opened.
These envelopes add some additional cost to a mail project so we recommend you test them first. But they are popular for high end donor or event packages where the additional cost can be justified.