I don't think Sal-site (ambystoma.org) solicits donations. They are funded by the National Institutes of Health, and their interest is supplying animals for research (usually medical research, not research into axolotls themselves).
I'm not really impressed with Lotl-love. As mentioned, caudata.org donations are for the support of the website. Caudata.org did have a "grant" program, with donations from hobbyists being used to fund research, but it takes a lot of time to administrate that kind of thing, and it isn't being done currently. You can see the lists of applicants for those grants in the General News section of the forum.
There are several very good organizations that are involved in conservation of wild amphibians (though not specifically axolotls). Two of my favorites are
Amphibian Ark-Saving Endangered Amphibians from Extinction
Tree Walkers International
Both are registered 501(3)c organizations in the US. This makes your donation tax-deductible (if you care), and gives you some assurance that they have reasonable financial practices.