I used to sell seeds and plants on Ebay. Make sure you have enough room for the plant to not get crumpled. They all seem to toss boxes around a lot. I water them well ( but don't drown them ) before sending so they don't dry out. Use something recyclable for inbetween for protection so they wont be banging up each other. Each pot ( mine were fairly tiny) I would tape it criss cross across each pot. Make sure they are labeled as to what plant they are, so they will not be confused.
And there is another way, using a ziplock or sandwich baggie. Make sure it has dirt for it so it wont go into shock, water enough so it wont go dry. Tie off the bag ( but not too tight for the stalk ) so you don't lose the soil. You can tape these in a laid down on the box inside. This will keep them In place so they wont fly all ober the place while being delivered. And sometimes I would write on the box, " this side up please." ( check with states to see if they are allowed ) California and Florida have tight restrictions on plants.
Make sure you tell the customers you are not responsible for how they grow them..if they dont follow your directions .. ( meaning, if they don't have a green thumb and they die..it's not your fault. ) Ship on a Monday preferably so they wont sit at the post office over the weekend ( if you mail to them ...most wont because they dry out there.) Or they will arrive in good condition during the week at a house.
Hope this helps some. I never had complaints.
I used to send by usps. They have pretty good boxes... a flat rate box is good. You can put enough in it, but there IS a maximum weight.
They used to have a longer triangle box for longer plants. I'm not sure if they still carry that one. You can also go online there to look at boxes and things for sale. Even the tape.
I used a weight scale. And did it online. You just enter info and they will send a shipping lable for you to print.
They act like that is new now, but I was doing that many years ago for free.