Everything you need for baby and nothing you don't
There is nothing worse in the world of new-parent retail than timing your day around an errand, packing yourself and the baby up in the car, sitting in the car while Ethel finishes the inexplicably long nap she took because you were driving, schlepping the car seat and your diaper bag into the store and discovering that they are out of whatever it was you came all that way to collect. Be it the only formula your little one will deign to drink, the exact toy her little cousin has asked for for his eighth birthday or an exchange on the hideous snowsuit Aunt Miriam sent for Christmas, call ahead. Even if they, “always have it,” have the person on the phone check the floor to make sure and, if at all possible, hold it under your name until you get there.
(This does not apply to, say, a trip to Safeway, of course. Although if you do call ahead and ask them to check on every item on your grocery list before you make the five-minute trip to their aisles, please tape the call because I want to hear their reaction.)
Related: order anything you can online and have the nice man bring it to your doorstep. Provided shipping is free or low, this is also often cheaper anyway.