This is a tricky shipment to do. If you have any doubts about the person doing the sending, don't do it at all!
First off, you should be very careful about the weather. Ship when it's cool, but not freezing. Consider the weather at both ends, and possibly in between too.
There are two methods that would work. My preference is to use breather bags. These are air-permeable bags, and you'll need the largest size. I'm not sure where to get them in Canada, but they are always being sold on AquaBid. You put in enough water for the animal to be comfortable, and no air space. Each axolotl should be bagged separately, and double-bagged with two breather bags each.
The other way is in large ziplock bags, with half water and half air. Double or triple bagged, and sealed shut with lots of tape. Remember that on airplanes there are pressure changes, so don't fill the bags all the way with air, leave some room for expansion.
The bags should be nested in soft packing materials and boxed. If the weather is warm, you'll need an insulated box and cold packs. And it helps to start the animals in refrigerated water.
All the water and cold packs will make the package very heavy, so expect to pay a bundle for shipping. When I shipped two adults, the package weighed over 4 pounds (2+ kilos).
