The Green Cabbage Trading Scheme provides (at a cost some may argue) collectors with opportunities for limited edition rarities, typically just 100 or less are available. These offers have provided four covers with the 10p Morporkia green with both overprints and different print varieties.

A series of covers were inspired by a section in Making Money, when Moist has been summoned to Vetinari`s office and wrongly assumes that it is due to damage to some of the mail.
We got carried away, said Moist. We were a bit too creative in our thinking. We encouraged mongooses to breed in the posting boxes to keep down the snakes ....
Lord Vetinari said nothing.
'er ... which, admittedly, we introduced into the posting boxes to reduce the number of toads ....'
Lord Vetinari repeated himself.
'Er ,, which, it's true, staff put in the posting boxes to keep down the snails ..'
Lord Vetinari remained unvocal.
'Er ..... These, I must in fairness point out, go into the boxes of their own accord, in order to eat the glue on the stamps,' said Moist, aware he was beginning to burble.
The GCTS cover involved was stamped Dribbled on by Toads

There are then three covers in a set Mail from Pseudopolis, inspired by a passage in Going Postal.
"We've heard the clacks has gone down again, Sir to Pseudopolis" said Aggy. "How unfortunate" said Moist. "Come Miss Maccalariat, come Mr Aggy - lets move the mail". There was a crowd in what remained of the hall. A cheer greeted Moist when he came down the steps. Give them a show, always give them a show. Moist commandeered a chair, stood on it and cupped his hands. "Special today, ladies and gentlemen" he shouted above the din. "Mail to Pseudopolis reduced to three pence only. Three pence! Coach goes at ten! And if anyone has clacks messages lodged with our unfortunate colleagues in the Grand Trunk company and would care to get them back, we will deliver them for free"'
The three covers detail the correspondence between Gerhardt Sock, Butcher and Tripe merchant, and Thrashers School in Pseudopolis. He wants his son to have a gentleman`s schooling and is willing to pay in named meats. Alas, it seems no schooling will improve his son`s intellect.
The three covers were said, on their release, to use the digitally printed stamps left over from the early days of Discworld stamps. This is NOT entirely correct as you will see.