In the past, I have replaced eggs for shipping, then once or twice, sent them at my own cost. So far, all my buyers have been very reasonable and didn't expect replacement. I wrap then individually in heavy bubble wrap with special attention given to the box corners and sides. I've had only one egg actually broken that anyone has told me, though one or two have been cracked. The broken one had what looked like a pencil punctured the side of the box. Tori in Alaska had one with a hairline crack, but the box was crushed on one corner. I agree that you pack so a truck could run over it and they'd be reasonably safe anyway.
This year, I will probably not be sending replacement eggs unless they candle clear/never develop at all. I did have a fertility issue with one hen this year and finally caught it when two people didnt get any to develop, then I couldnt even get any fertiles from her at home. Those I will replace for shipping. ThreeBoysChicks got a shipment of 19 eggs from me recently and ALL of them were growing at first candling. Great packing is the key; after all, it's the only thing besides the health of your breeders that you can control.