Jeff gave you very sound advice, I'm going to suggest some things that might help you with the "particulars".
You probably won't be able to ship collect - i doubt you have an account unless you shipped a lot of reptiles prior to 2002. You can still ship, but it probably won't be collect - it will have to be pre-paid. (which might mean having the buyer send you cash prior to shipping)
They will tell you the exact price after they weigh and measure the box.
We used to use warm water bottles for heat (which work the best in my opinion) - but have switched to those 12 hour heat packs that don't use up oxygen. Water bottles add to the weight. They ship by volumetric weight, but with water you run the risk of paying more for the weight of the box than the size of the box.
I'm not sure what the price is like shipping from Pearson, but I know that the cost of shipping from Alberta to Pearson has rison from an average of $75 / box - to $120 / box.
There are some heat packs that do use oxygen as a catalyst. When we have used those, we've either used a lot of air holes in the container (which decreases it's insulating properties quite a bit unfortunately - bad in winter) One other method is to have a few air holes, but position the holes right near a place that you have taped a heat pack to the side of the styro box. We usually use 2-4 8-12 hour heat packs depending on the size of the box.
Make Air Canada confirm a heated cargo route all of the way. Even in Summer, temps up in the air can be as low as -40 c (or so I am told) and you don't want to risk an unheated cargo flight -ever.
The signs we put on our boxes say something like "live harmless pet reptiles, please keep box between 75c and 85c at all times" We put 5 of these on each box, one on the top, and one on each side. We also put our phone number on the box, and have had Air Canada offices phone us if there is a delay, to let us know that they have our pets in a warm place. Some staff are really great, some aren't so great.
We put our snakes in pillow cases, but also put bagged snakes in rubber made containers individually. I'd be concerned about putting 3 boas of that size in a box together without using containers (smothering, crushing, general stress).
Sometimes we tape the heat packs on to the outside of the rubbermades. You can just put them loose in the box. We fill the remaining space with those packing stryo peanut things. It helps retain heat, as well as provide a nice cushoned ride incase the box gets bumped around (which I think probably happens more than we'd like to think)
We haven't always done all of this, and have never had a really bad experience (dead snakes). We have had people tell us that the reptiles showed up cool - which is part of the reason we are so diligent with our new methods. You never know who is going to be handling your shipment, and there are some people that don't like any snakes, and would rather see them dead.
I just read what I have typed, and realize it might sound scary. I didn't mean to make it sound bad, I just wanted to share the tips we have learned over the years to help you with your shipment.
If you have any questions about the methods I have tried to explain, feel free to email us - ScalesZoo@sasktel.net