Duck eggs are larger than chook eggs and can vary in size. It stands to reason that the bigger eggs will produce bigger ducklings, and the smallest will produce the smallest ducklings. Being less robust, the smaller ducklings are more likely to be trampled and overpowered for feed and water. Hence, I put the smallest eggs in the fridge for eating and any medium for my incubation, so the fertile Khaki Campbell duck eggs I sell are more likely to hatch into robust ducklings.
Because the fertile Khaki Campbell duck eggs must be fresh when incubation starts, and because the eggs I sell in the post or for delivery are likely to be a few days in travel, I send the freshest eggs to buyers. The few extra days that may delay my starting my next incubation, should not reduce the probability of the eggs hatching.
I charge $5 [five dollars] per egg [$60/dozen] and give a $60 discount for an order of four dozen. I usually send by Australia Post MyPost Express Post, or parcel post; your choice. Parcel Post is a little cheaper. I pack them into specially designed polystyrene blocks to protect the eggs [as per photo on Profile]. They cost me $5.50/each per dozen eggs, which I include in the cost calculation. Australia Post charge $13.95 to collect the parcel so I take it to the post office, and include that in my Packing and handling charge of $15 per order, additional to the $5.50 per dozen block [they are reusable when you are selling eggs].
My Postcode is Q4304 which is just inside the Brisbane Metro postcodes so will reduce the postage a little compared to my being regional or rural in some cases - zone 2 instead of zone 3 in some cases.
I know many people are dissatisfied with Australia Post. I am dissatisfied with AP because they will not "deliver". To them, "delivery" is to the maybe nearest post office. I am investigating an alternative carrier, which may be a little more expensive, but probably worth it to most people for proper real "delivery" with certainty. That will by the buyer's choice, if and when I have such a carrier