it's getting harder to find the pre-printed-on-the-back postcard packs, although plain paper is fairly easy to get hold of (by plain paper I mean not pre printed as a postcard - you MUST use the correct paper as supplied with the ink ribbon). Last time I costed the prints, they worked out at under 20p/6x4 print - cheaper than buying postcards or getting small quantities of prints done on the high street.
I didn't know Canon produced plain paper for the Selphy series.
All the packs I have bought have been pre-printed with the Canon logo and postcard markings on the reverse.
Academic point really, since I've never sent one as a postcard, and if you did, you could still write an address and stick a stamp on the non-printed side.
The cheapest place I've seen recently for the paper/ribbon packs is 7DayShop although it looks like they are now appear to be charging for delivery, which puts the cost up.
I always buy the 108 sheet triple packs, which work out cheaper per print than the 36 sheet single packs.
I did also recently see a good price advertised for the 108 packs on a certain on-line auction site.
One thing I always wanted to try was the so-called "panorama" paper for the Selphy, which was 100 x 200 mm (4 x 8 in the old money) but these no longer seem to be available.
I guess lack of demand has caused them to be discontinued.
Another point I also noticed is that the latest (CP 810 and later) Selphy printers appear to use a different paper pack to the older models.
Maybe that's a sign that those of us with the older models should stock up on paper/ribbon packs now before they are phased out.