Jenny2U,
Thanks for the posting--I'm an avid (some would say crazy) Newfoundland only collector. A few more details to amplify the information above:
I have this cover and in my opinion it is a nice to have. Value is maybe $25 to $75, perhaps $150 if a rarer version (see below).
As noted above : this is an unofficial First Flight Cover (FFC) by a private airmail company and some were posted on later (i've got one that was posted on to Washington DC, wierdly). The mail was seized and delivered later with the philatelic marketing note signed by the President. WT1's post gives more detail on this.
Historically this cover was sort of officially noted in two books of import (The Airmails of Canada and Newfoundland (552 page detailed book you could buy for $35 or find at a library) and also Walsh/Butts (Newfoundland Specialized Stamp Catalog (aka NSSC), a 489 page book covering all Newfoundland stamps. Both the AAMC book and Walsh reference and list this cover as FF48 (AAMC) or FF28 (NSSC). I provide these details for two reasons : so you can note the specifics on your record or the cover in pencil but also, so you and the community can see that at one stage these covers were sort of officially recognized. There is controversy though and some purists don't like this; in fact Walsh's NSSC has since deleted it from their listing and has a footnote on it, but considers them "souvenir items". I don't disagree but also think that they have some value and historic interest. After all the covers did fly on an "airship" in 1931.
Lastly, there are actually 4 different shades of that funny little label on the cover. (I can't bring myself to call it a stamp, because it definitely not in Scott or Unitrade). Yours is a plain yellow one with red ink cachet. There are also 3 other variants and are sort of rarer or harder to find. (A yellow one with vertical stripes, an off white one with the ink cachet in black not red, and a green one with green cachet.
If anyone wants to see these images, let me know.
Here they are :



