There is also ebay and paypal fees, and even on BYC most sellers bought a golden or platnum membership, . So theses fees have to be figured in your cost also.
I also think everyone should send extras. I check fertility about every 2 days (my spoiled chickens think they NEED treats) Even if I have 100% fertility in the ones I check , obviously I can't check the ones I ship, so I always send extras, and I really don't like it when I don't get any extras.
I bought some eggs from a BYC'er got 100% hatch, bought some more of the same eggs, none hatched( none even developed). I also believe alot has to do with the shipping. Most of the time I do not think it is the seller fault, unless they pack poorly.
I use the 12x12x8 boxes, I know alot of people have good luck with the smaller ones, but I try to pack like I would want them sent to me. I hate when people put them in egg cartoon, I've gotten more broken eggs and worse hatches when shipped in them.
I have always tryed to leave follow up feed back when I got decent hatches. But sometimes I have forgotten to do it. I always look for it to see how their eggs are hatching.