Not sure if this item works with ALL Canon Selphy Printers but I just brought some of these for my Canon Selphy 720. Postage is just £3.00 for up to 4 packs, so now I have enough for 432 prints.
Brings the cost down to less than 13p per print!
Not as cheap as on-line digital photo processing companies I know, but darn cheap for home-print processing.
By far the cheapest price I could find on-line for this (even off eb*y).