Shipping Priority mail, which is how most day olds ship where I live, it is done by weight. It is very easy to calculate prioity mail shipping at USPS, if you know how much your babies and their packaging weigh. As far as shipping older ducklings, I have no idea what the express shipping would cost, but I know it is alot more. And the day olds I received last spring were just in a small, not particularly sturdy, cardboard box with small open air holes in it. And I received them from a very well known hatchery. I think figuring out which post office they will need to be picked up at is the hardest part. They usually stop at the local hub and mail carriers almost never home deliver, but each city is different and so is each hub.
Shipping a 3lb 8oz package priority mail is $10.80, I know this because I just mailed one this morning and asked about ducklings while I was in the PO. The best way to find out what the particulars are on duckling shipping in your area is just to ask the post office. I hope this helps.