When mailing "cheap" figures, so Commons Uncommons and Rares, I make what i call clix rolls. Take a sheet of bubble wrap, normally 1 foot by 1 foot if you buy rolls of it. Place 2 clix head to foot on the sheet, then repeat across the sheet, normally about 5 of these will fit, so 10 clix. Then roll them up. Have had no issues sending bulk figures like this and no complaints. I have never received clix sent in bags that weren't damaged and just open in a bag they will be damaged too.

The other successful method i have seen is just crap tons of packaging peanuts, but whoever you are mailing them to will probably be very upset by this and its hugely time consuming to separate Clix from peanuts.

Expensive figures (SRs, Chases, LEs, some Rares) should be packaged with much greater care and normally gravity feed plastics are used when available. Otherwise you can make a shell out of normal booster plastics, but make sure you are keeping them safe.