The last line, spoken by the late Terry Pratchett (as a postman), is "That's a bit of an embuggerance". This is a reference to Sir Terry's diagnosis with Alzheimer's disease, which he always referred to as 'A Wretched Embuggerance".
Not being a professional actor, Terry Pratchett apparently had some difficulty in his small walk-on role and, in his own words, "needed six takes to master the tricky task of walking across the room".
When Moist is giving his speech to the press, unveiling the stamps, you briefly glimpse the recurring character 'Otto Von Chriek', the Vampire photographer who first appeared in the novel 'The Truth'.
Adora's three rooftop accomplices, the 'smoking gnu', are a blatant reference to the three members of 'The Lone Gunmen', Byres, Langley, and Frohike, from the TV series 'The X-Files, who's characters proved popular enough to give them their own series, 'The Lone Gunmen'.
Although he appears in the book, Death does not appear in this miniseries.