I bought a bunch of cartons and put my own labels on them. I have a small sticker that is also on the carton that says please return the cartons at your next purchase or something to that effect.
It doesn't add that much to your expenses and you are reusing them.
I know some people who give a 25 cent discount off of each dozen eggs for each carton a customer returns to her.
Now, on another note, there are some other threads here that state its not legal to use supermarket eggs to sell your own in.
It says you can reuse your own but not some that other eggs came in.
I am under the impression it was for contamination but I am not completely sure its not to do with using someone else's packaging to market your own.
I would do a search on this and see what you come up with.