From my experiences, you have to pay before anything gets shipped. That is the case with pretty much anything you buy online. If you buy something from Amazon, without Prime, it may not get shipped for three or more days, depending on the vendor/merchant.
If I pay for something with PayPal and then never get my purchase, I can open a case with PayPal and ultimately get refunded if it's actually a fraudulent attempt on the sellers behalf. I've had this happen once.
In your case, I would not ship anything until payment is received. If someone doesn't like that, well then, they'll just have to buy snakes at Petco or an expo. The way I see it, when I buy something from someone, especially like this, I fully understand the seller has a life, and things to do, just as I do. I would expect the seller to send me notifications if anything changed beyond what we previously agreed upon... or when my items shipped.
I think you're in the right here. Keep the snake until payment is sent in full, then ship when it's best for you and the snake.
Just my $0.02