Here is how I do it.

Get yourself down to your local supermarket and buy a cheap plastic
food container with lid. I use a Tesco 330ml one.
Only 45pence!

Take container and put a few air holes in the lid only.

Put in the container some damp cotton wool under and over the snails
(or some paper towel) to line the inside with and to stop the snail
being thrown about.

Put container in a jiffy bag and send 1st class post.
If the inside of the bag gets wet, you've used too much water.

The snails don't mind being out of water as long as they are kept moist
and warm.

They tend to go into a sleep when sent like this.

When your friend gets them, tell him to take the container, remove the
lining and fill it slightly with the tank water so the snails are *not*
covered.
Turn the snails so that their trapdoor is visible.

Place container in the tank with the lid off so that it floats.

In a few hours or less the snails will 'come to' as the tank water
warms up the snails and they will move about and righten themselves.

They can then be placed in the tank directly without fear of them
drowning due to them being still asleep.

If you place Apple snails in bags of water then they can drown if they
can't get access to air perhaps due to rough transport.