I don't think I will ever have such funds for a cover like this, but it's wonderful to see it.
The stamp in Gibbons is catalogued at £500 [$800], but the cover , Philatelic though it is, would be worth more than you paid in the UK market I'm sure. Well done !
A few points about the cover [that I seemed to stare at for hours last night] and which I tried to research a little when the drooling had subsided.

I wonder if the red cancel had some significance? Many big Exhibitions of old came up with red cancels.
Also, I wonder why the maker did not place the Air mail etiquette further down left side [above the Show label], so the show logo could be displayed ? In a hurry perhaps?
Lastly about the stamp and venue city. I cannot believe that many big International Exhibitions were held in Le Havre. I will try to look that one up. I found it unusual.
I collect Philatelic Exhibition material but non-postage [unless they happen to collide] and very often special items are issued in very small quantities taking up the price. Frustrating for a collector of my topic

So here is a stamp, overprinted for the Exhibition but in obviously small quantities. I find that a little strange given that it was an International Exhibition and the 'souvenir' was postally valid? I wonder how much they cost at the show ?
More research is needed.
Thanks for showing it anyway. It really caught my eye.
If I come up with anyhting, I'll post it here.
Londonbus1....will never be the same again !