Caveman,
Shhhh! Don't go putting dangerous ideas like that into their heads.
I have even better ones ;-) You could do it like with calling cards / mobile phones. You buy the card, scratch it and get the code, enter it et voila, instead of 30 minutes of talk, you get 24 exposures. Or you may buy professional cards for 36 exposures, or XL packages of 240 and XXL of 360.
Naah, it's too obvious. We can hide it. After a number of exposures, the camera blips and displays E734. You look in the manual and find out it means "Error 734 - service needed". Next time it's "E345 - shutter calibration required". etc. The guys don't actually have to service anything, they just take your money, reset a cmos counter and here you go for another round of shots.
cheers, caveman
