Legally, the envelopes serve no purpose. The term (if you want to ask a lawyer) is "form versus substance." If a reasonable person in possession of all the relevant facts would conclude that what's being sold is in fact an airline award, all the plain white envelopes and cute words about "free gifts" won't make a whit of difference.

The airlines know this. They do whatever they'd do if the envelope wasn't involved. The envelope doesn't stop them. AFAIK nobody has ever used it successfully as a defense.

I don't see why anyone bothers. Then again, I don't sell airline awards, either.