I have no experience myself; I only ordered ducklings once and that was last May. But I recall last winter reading odds and ends of sad stories on BYC of birds that arrived late or never, due to weather or post office or even shipping error. I'm sure that statistically most animals make it to their destination fine, but that doesn't do any good for the ones that don't. I think if you order this time of year that you are really increasing the chances of having problems. I didn't see where you're located but f course it's bitter cold in much of the country and nasty snow and ice storms with all their delays are happening in many places. While a heating pad will help out, it won't do any good if ducklings are sitting in the back of who knows where for a week.
I know it's hard to resist those cute little duckies, but do yourself and them a favor and hold out until spring. Then you can relax and even spend some quality time in the sunshine with them. Think, but only for a second, how badly you would feel if you opened a box of casualties.
Just my .02. That's so sweet somebody gave you a gift certificate. I don't think it would even occur to anyone in my circles to do such a thing, although I have been the lucky recipient of many other forms of generosity.