I suggest using 100 percent oxygen, not just natural air, especially if using an overnight mail service. I also suggest using a cargo shipping service to get package within hours vs days.
I no thst what everyone suggests but I have no airport around here that carries cargo and I have also had fish mailed to me through fedex and ups ... Those r 100$+ for one day I have also had fish mailed usps 2 day and they were completely fine no issues whatsoever and it was 20$.... I wouldn't suggest that shipping type for myself if I didn't believe they would be ok