I see no rule that says an adult cannot be mailed
https://faq.usps.com/articles/Knowledge/Parcel-Size-Weight-F...
Adults are too heavy.
So you can order chicks by mail, but you can't return them by mail (unless birth + packaging + USPS shipping, including delivery + return packaging + delivering return package to USPS for return mailing all takes < 24 hours, total, I guess)
Babies wouldn't be much different. Just need bigger air holes in the box.