I would just ship the plants in whatever they've been growing in to this point.

Switching up potting soil on plants prior to shipping might put ADDITIONAL stress on the plant.

A few days travel should not be enough time for your plants to dry out.

When I've shipped, I've ...

Made sure that the soil is moist ... not wet, and ...

Secured the soil in the pots with cotton balls taped around the plant's base.

Wrapped the entire plant and pot in layered newspaper to create a appropriately plant-shaped space which will protect the foliage.

Finally, minimize, as much as possible, movement of the wrapped plants in the shipping box with additional crumpled newspaper and/or shipping peanuts.

All to whom I shipped have reported receiving their plants in good shape.

OTOH, when I have RECEIVED plants that incurred shipping damage, ... it was sometimes evident that the sender had shipped them in fresh potting soil (where the plant simply did not have as much rooting into the new mix, and was, therefore, not as secure).

All that said....the only thing I do that eliminates alot of paper cotton...plastic wrap...I use the small pasta containers that you can buy at the grocery store..they come in 8 and 120z and the 80z if it is a plantlet is perfect...I just wet a paper towel wrap the roots and put it in and the height of the container is just enough to hold it right in place then you dont have to worry about it rolling around. But this depends on the size you are shipping..theres my two cents.

Yes, I am afraid the plants I will be sending her are also going to have been recently repotted. I guess, when I repot, Ill just use what I always use as far as potting mix goes.

Im going to research the various theories as far as the mixture rate of potting mix goes. For a long time, equal parts of the three were considered good. Now, there are some changing theories afoot, I suspect primarily having to do with the increased popularity of wick watering.

For the sake of conversation, and with the stipulation that both vermiculite and peat absorb and hold water, if you wanted to reduce water retention, would you eliminate one, the other or a little of both?

My potting mix (I wick water) is 1:1 peat and perlite, no vermiculite at all. Vermiculite can be helpful in some watering setups and growing conditions, but I know several people who don't use it at all. I live in a very humid climate, so water retention isn't a concern.

Hmmmm. The pendulum is starting to swing toward the reduction or elimination of vermiculite. Very interesting.

I don't use vermiculite either ...

I always wondered how sellers went about this conundrum. Thanks for the info.

It is better to send an established plant than recently repotted. Any kind of soil will work if you pack the plant when it is barely moist, not dry and not soggy. Some sellers dry the plant a bit so it is less crunchy - and add a little bit of water to the pot before wrapping. Wrapping the pot in Saran wrap preserves the moisture and keeps them hydrated through the trip and prevents the plants popping out of their pots. Very important is make sure that is the plant is not moving and bouncing inside the box. I wrap them in 4 layers of a newspaper, sometimes more and secure it so it creates a wall. The newspaper or a cardboard should be secured to the pot, not to the plant. You need to fill empty space in the box with wads of paper, packing peanuts or air pillow type packing materials - so when you are done - you shake the box - nothing moves there. If the temperature is below 84F, you can risk sending plants without a special insulation -bubble wrap and layers of newspaper will work. If it is higher - you need insulation - styrofoam inserts you can cut to the size of the box - or fiber wall insulation. If it over 100F - it is probably worth to wait until the temperature goes down - or you can arrange the box to be picked up by your addressee from a post office instead of being delivered to the door. USPS.com has this option.

Live plants should be sent priority, not first class. And there are some areas that give you shivers when you send the plants. Fayetteville NC is one of them. The plants come cooked to mush even when it is not that hot. My take - they keep the parcels for a day in these big trucks before they get to the rural post offices - the temperature gets to 130F inside. No amount of insulation is going to preserve them.

Winter shipping is so unreliable - I do not risk it. Heat pads do fail both ways.