น้ำเงี้ยว . . . All I know is, given the context, was that person A was going to mail it to person B.

Google makes it appear to be a food . . . but you can't just mail food . . .

so I feel like its some kind of snake spice in a jar from Isaan or something . . .

The base meaning is certainly a food. See http://gotoknow.org/blog/nidnoi/121898 . Perhaps there is a slang meaning as well.

Yea the first thing I did was a google image search, but it had an endless variety of foods that appeared to have no connection . . .

Key Informant/Significant Other says it is a northern Thai (curry-like) topping for Khanom Jiin.

Probably right, as person B spent half her life near Chiang Mai, and google liked to include kanom jeen with it.

So . . . what exactly goes into it? Snake?

It is indeed a topping for Kanom Jeen. I have given you a link to how to make the curry paste and the dish itself. The picture at the very bottom gives you an ideas of what it's supposed to look like finished.

It's pretty tasty too, my ex used to make it a lot and fortunately since i've moved south i've met someone here, from up north, who knows how to make it.

Never had it with snake but i'm sure you could use it as a substitute.

Comes from the flower of this tree. No snakes involved.

I've just realised, i forgot the link.

http://www.nuihome.com/?p=812