I can understand them wanting candled eggs. People have cracked eggs and gotten partially formed chicks. Should that ever happen? No, but it has.
Not everyone wants to crack open an egg and see blood spots or meat spots, either. That happens a lot more often and you can't do anything to prevent that from happening, you can only cull after the fact, if it happens with particular chickens. Blood spots and meat spots are one of the things people have traditionally candled for, in addition to chick development. It's why commercial egg operations candle. Some people could care less about this issue and some people are pretty phobic about it.
For a dollar a dozen, though??? Heck, they should be gathering their own and using their own cartons, for that price. You could offer candling as a service with an added fee or just let them get their eggs elsewhere.