First off Welcome to BYC..
What a precious lil one too. When they arrived did you make sure to dip this ones bill in water as well as the rest? I would separate this one and one other by them selves so this lil one has a chance to recoup the trip and having a buddy always helps and being separated will keep it from being trampled. Do you have any poultry vitamins? if not a trip to TSC would be a good place to pick some up Poultry Nutri Drech is a great one because yoiu can give it straight into the bill, not down the throat but tiny amounts either using a small needle less syringe or just dipping the bill. But this goes straight into the blood stream not through the gut like most vitamins do. Please keep us updated on your little one. Shipping is rough on them and being jostled around and being trampled is rough on them too, hopefully with a little time and some TLC your lil duckling will be up and running around with the rest.
In case these are your first ducklings
https://www.backyardchickens.com/t/750869/raising-and-caring-for-ducklings#post_10611711