Ducklings that are shipped can often develop fluid, and electrolyte deficits, especially if there was a problem during the incubation, and hatch such as improper absorption of the yolk sac. Along with excess heat, cold, and rough handling this can send ducklings, along with other baby fowl into a poor state.
We can help by keeping them in a warm, quiet area, and ensure they are not getting bullied by the other birds.
Administration of a poultry vitamin such as Sav-A-Chick, or Nutri-dench will often also prove useful. Warm mashed scrambled eggs can also be given.
https://www.tractorsupply.com/tsc/product/nutri-drench-poultry-4-oz
Ducklings lack peristalsis and aspirate on fluids easily so care must be taken to give small amounts of feed, and liquid at a time to avoid aspiration of the material. Soaking a cotton swab in the liquid and wiping it along the bill may be the best way to do so.
Due to size, this is the brink of what we can do for ducklings dealing with a problem like this.