She was not thrilled to be in the carrier, so most of the drive, I had one hand in the carrier (it has zip closures, so I just opened it enough for my hand) trying to placate her. When we got there, Melly, in her carrier, went into the kid seat of a shopping cart as we navigated the aisles picking up her new goodies: some food, dental hygiene chews, enzyme spray and her very own scratching post!
One thing we just discovered...even at her spazziest, when it seems that all she wants to do is run and attack the pattern in the floor tiles, chase her own tail and explore any new thing that crosses her field of vision, there seems to be one sure way to calm her down. I jokingly stuffed her into my lightweight coat, expecting her to wriggle and escape in short order. I was wrong! She snuggled up close and relaxed, making NO attempt to get out.
So, after torturing her with the incarceration of the cat carrier on the ride to PetSmart and then in the store, I decided to try the snuggle in the coat approach for the drive home. I tucked her in, with her head peeking out and my arm around her, snuggling her close. I also plotted a course home that avoided major roads, so that I could pull over and stop if she made any move to escape. Those precautions proved unnecessary as she contentedly stayed right there in my coat the whole time. (If this is a huge no-no, please forgive me and be merciful in your comments.)
We returned home and I tucked Melly back into her room with her new scratching post, which, I don't know if she's scratched yet, but she seems fond of it.
When I came out of the bathroom, Scarlett was standing out there, peering down the little hall toward the room curiously, but scampered away when I closed the door behind me. A few minutes later she came over, and, after sniffing my hands and NOT showing any adverse reactions, suffered me to pet her a bit. She's now happily perched on her roost at the top of her tower; apparently unconcerned that Melly had been on it earlier.