I show up at the post office with honey water and mashed hard boiled egg. Open the box up, make sure everyone has a good long drink, take the water away, and give them the egg to munch on on the way home. I keep those chemical handwarmers in my van normally, so if they look chilly, a few of those go in the box as well for the drive. In the brooder, I have Mama Heating Pad set up and scootch everyone under it (since they've had a snack). Got Poultry Nutri-drench in the waterer for the first 12-24 hours, then switch to plain water. I feed fermented feed--it is a probiotic, so don't need to add that. I generally only need to check for pasty butt the first few days with this routine because the shipping stress is overcome quickly and heat stress isn't an issue.
I do also start feeding finely minced fresh herbs/greens/veggies with sand grit as treats after their first week, making sure to include hot peppers weekly. Occasional seeds and bugs. It gets them eating a variety of foods from the start (and the capsaicin from the peppers stimulates gut immunity) so they aren't so picky later.