I made a trifold that I tape to the carton with each carton that I sell to new customers. I just touch on the freshness aspect - I figure if they didn't know how much better these eggs are than store bought they wouldn't be at my door in the first place, so that's kind of beating a dead horse. I do talk about the care the girls get, with a couple of photos of the coop, and the fact that they are allowed to free range in the yard as much as possible.
I also make clear that I do not prewash the eggs. I tell them to do that just before they use them. And I bring up the ugly facts about the possibility of an occasional blood spot or meat spot in an egg - what they are and what they are NOT, and that although unappetizing they can be safely eaten, with or without removing them. I think that one factor grosses people who have not had backyard eggs the most - it still gives me the heebies and I know what they are and I'd hate for someone who has never encountered that before to be surprised by it! I don't sell eggs by the dozen. I sell by the 18 pack. I reserve the dozen sized cartons for our home eating eggs and for eggs to give away. Most of the time folks who have been given a free dozen will come back for more later.
When I have pullets newly laying, I know that the egg size in my cartons will be all over the chart. I don't like to take the time to put all the like sizes of eggs in each carton because as each egg comes in, I write the date on the fat end of it. I also pencil in the dates of the respective eggs in each carton. So I always toss in a free dozen when I have a lot of pullet sized eggs in a given day's carton.
Now, mind you, I'm not going to make money this way, and I don't care. I don't even care if I break even on the feed, although that does happen most of the time. I live in a town of 600 people and there is a chicken coop on almost every block. So I compete by keeping my price low, throwing in those free dozens when the eggs are small, making sure that I acknowledge and explain that not every egg will be perfect inside, and by making sure they know that the eggs they are getting are unwashed. I like the way I do it, and as I'm getting more and more repeat customers it's become clear that by having a clean environment for the chickens that folks can see from the street and nice looking eggs I come out ahead. I even have two customers who just come into the house, get what they need out of the fridge, and leave a post-it note with how many they grabbed and the money on the table when we are out of town. Works for us.