You certainly are posting in the correct place, welcome!
The original postcards where given away randomly with most of the highfield mole books along with the bookmarks and t-shirts.
There was 500 sets made in total so to have a full set together in the wallet is rather rare. The release Code is MaS 4 with the other MaS numbers being the following items:-
MaS 1 - The Highfield Mole - The Circle in a Spiral. First Edition Hardback with an insert of eight colour plates. Print run of 500.
MaS 2 - The Highfield Mole - The Circle in a Spiral. First Edition Paperback with an insert of eight colour plates. Print run of 2,000.
MaS 3 - Bartleby T-Shirts - issued in a pre-launch promotion. 100 produced.
MaS 4 - A set of six postcards in a bespoke wallet
MaS 5 - A3 Art print of Bartleby
MaS 6 - A3 Art print of Rebecca
MaS 7 - A3 Art print of the Crawfly
MaS 8 - The Colony leather bound edition of the hardback. Limited run of ten.

Worth? hard question really. Ive never seen a set sell, only singles. So a 'fair amount' I'm sure. But as always, its only worth the amount someone is willing to pay for it.
Another thing to consider is if the post cards and wallet are signed and if so are they signed in the original 'Gold Pen' which was the case for the first promo items.
Hope this helps