Sounds like you did get took since they won't send you the remaining dozen or refund your money. When I get money from an individual for a bird/birds then it goes into my savings account until they birds have been shipped and arrived safely at their destination. Until that point then it is not really my money. Sounds like these people are low on cash and dont have it to refund or are just flat out dishonest.
As far as the cracked eggs, ruptured air sacs and low hatch rates, that's why I quit selling hatching eggs on wild waterfowl. I took the time to only select fresh eggs, pack them in bubble wrap and then pack them in hay and I still had people complaining because eggs didnt hatch or there were some cracked ones or because the air sacs had ruptured and they died. I was even accused of taking infertile eggs out of the incubator and sending them to people. Now I just sell live birds and I ship because for the same price you paid for that one call duck I could have had a pair of Mandarins shipped to your local post office already grown and ready to breed next spring. Long story short, unless you are picking up your hatching eggs yourself then there is no way you can be sure of the conditions in which they were shipped and it is impossible to hold the seller responsible for low hatch rates since he/she is also not in control of your incubation process.