I would just write fragile that's what the owners of that snail wrote when they posted them to me and it's a good idea I wouldn't think writing live snail would make the postman/woman would be very happy lol...
You'd be surprised! I received a parcel with the words 'Live Cockroaches - Handle With Care' written on it, the postman knocked on the door to tell me that it made his day, and went away having learned about keeping them as pets!
The holes punched into the container should be punched from the inside out so snailies don't hurt themselves. The best food to place in there would be something that would get a little hard or rubbery and not rot.
3 species unidentified slugs
What would happen if I just package them up in a box, well hidden within another box and do it without declaring?
You would be breaking the law and both you and the recipient would face potentially enormous legal consequences if caught. Australian customs and border force are VERY strict and thorough. It would be an incredibly stupid and irresponsible thing to do.