Are you sure she said litter box trained? The pup is very young to learn this and how she would even go in a litter box at that age is surprising. Some people put a pad in a litter box or on a tray, maybe that's what she meant? Congratulations, on your new puppy! One of the things I bought before I got Joey was a car seat that I used as a kennel next to my bed during the early months. It has a zipper and I could place my hand in the kennel and calm Joey down. The first month or so, they have to pee during the night, so they should be in the same room as you, so that they will wake you, when they have to go. Place them on a pee pad, let them pee, and give them a small treat like an unsweetened cheerio, and place them back in their crate. The crate should be small enough that they won’t pee in it, and just big enough so that they can stand up and turn around. This is the type of carrier I used:
Snoozer Roll Around Travel Pet Carrier - Pet Carriers with Wheels - Pet Travel Center. I still use it for car travel, a metal bar attaches it to seat belt. Have fun shopping and get sleep while you can!