Well, you should have known the cost of shipping ahead of time, at least an estimated cost. If the sale ends up being a break even, loss, etc. that's 100% your fault not the buyers. If you post a snake for sale, agree to include shipping, then it's a deal and you should follow through. Small price to pay for a lesson learned. What it comes down to is your word, your reputation. What's your reputation worth? What's your word worth? Accept responsibility for YOUR mistake. Obviously the buyer should get their deposit back.
I'm sure that my direct honest answer is not what you wanted to hear. This was a missed opportunity by you. You had a chance to show your level of integrity. Imagine if another buyer goes to that other site and reads up on you. Seller backed out, failed to honor the sale, etc. It could have read, seller underestimated shipping cost, resulting in loss but he honored the sale. Standup honest guy. Received a great snake, healthy, well packed, shipped properly, etc. If you were researching a seller how would you interpret these two examples.
Accept responsibility for your mistake, take the loss, it's not the end of the world and seize the opportunity to gain a new customer and a much more positive post about you on that other site.
Just my .02 cents
PS..Just a suggestion, it works for me, "buyer pays actual shipping cost". We get our boxes for free most of the time, the stuffing is cheap/free, and it's not much trouble or time to fill out paperwork and take the box to the carrier. I hate it when I see sellers making a profit on shipping.