Mail-order calathea arrived after six days in the box, half the leaves yellow — repot now or leave it alone?

Do not repot. A plant that has just spent six days in the dark with no light and no airflow is running on reserves, and a repot removes the working roots at exactly the wrong moment. Unwrap it fully, let the surface of that soaking mix dry, put it somewhere bright but out of direct sun, and leave it entirely alone for two to three weeks. If it is still alive and pushing anything new after that, then repot into your own mix.

Contact the seller today with photos, before you do anything to the plant. Most decent nurseries have a claim window measured in a day or two from delivery and want photographs of the plant as it arrived, packaging included. I have had two replacements and one partial refund that way, and the one time I waited a week to complain I got nothing.

One exception to the leave-it-alone rule, and it is the reason to look before you decide: if the mix is genuinely waterlogged and the roots are already brown and mushy, waiting three weeks means watching it rot. Slide it out of the pot without disturbing the root ball, look and smell. Firm white or tan roots and a normal earthy smell means leave it in the mix; slime and a sour smell means the repot is the rescue, not the risk.

I repotted a shipped plant the day it arrived, into fresh mix, because the nursery soil looked awful. It died in eleven days. The next one I ignored for a month and it is still here three years later.