I kept it in its original container, which had only one pin-sized hole for ventilation. It was doing find for the week, and the seller said it was enough, so I left it that way. Since I was driving, the sunlight was on/off. I was misting the container. The temp was probably ~78. No less than that. The ride wasn't super bumpy, but it wasn't 100% smooth. It was on a soil-like substrate. I think the seller said peat moss or coconut. The abdomen didn't look small to me, and no. I was going to take pictures once I got home...but that didn't work out.

I don't think it started molting - it was soon after it finished a cricket, and it wasn't upside down. That and I waited a couple days, and it still wasn't moving.

I had the AC on. Maybe it was too cold?

If, in the future, I get another, what can I do to prevent this from happening? How does one usually safely transport a spiderling?