I don't make mega shipments but I like to go with an inexpensive thermal bag. They come in all makes, sizes and colors. They fit nicely into shipping cartons with a little newspaper to keep them from rocking around during shipment.
I buy plastic storeage containers for shipping non-food stuff. I tape the lid down and wrap it with brown paper.
I also like to put little Knic/Knac gifts in my packages. Sometimes there is a joke in there too.
I had a relative who left a beloved flipflop behind and asked me to send it. What was the challenge in putting it in a brown envelope and mailing it?
I noticed various items left behind by other guest as well. (pj pants, t-shirt, ball cap, cell phone charger, pr earrings) I put all these items in a container and mailed it to her with a list of the other people and their addresses. She could mail each person their stuff or mail all the stuff to the next person on the list and cross them off. Just to say, everybody mailed a package because she accidentally put her flipflop in the box!