Even with the species specified in the contract being wrong, I don't see it as a relevant issue.
There should be absolutely nothing to be conflicted about.
This is no different than if they called you up later and said that they had undercharged you, and you needed to go back and pay more. You don't call someone back after a sale demanding more money, and you don't call someone back after a sale demanding additional concessions to an agreement!
The only reason this is in any way messy is because they are deciding to make it socially akward (spell??). It is expected that you will extend the curtesy of returning their phone call (you don't have to), and in standing your ground, you may feel yourself to be in the uncomfortable position of being "difficult" with the "nice" volunteers who only want to help animals... (They are counting on this, by the way.)
Gaah!!!
Animal rights types are some of the most full of themselves, dingbatty, irrational people on the planet. Many of them work for places like the Humane Society. You have unfortunately got yourself entangled with them. (I am phrasing carefully, and could say a LOT more, but don't want to trip the language censor.) Just say NO, and allow no more discussion on the subject of your snake. Hopefully, they will leave it at that. As far as I can see, they have zero standing in this.