I have sent Lava bars, but haven't gotten a reaction about them specifically.
They are tasty. Maybe it's because of the texture, but they remind me most of chocolate frosting.
Using the hot car in summer test, they do hold up in heat.
To be safe, the first time I put the box (I sent a box of them that I got at Sam's Club) in a large Ziploc.
Since then, when I've gotten them, I open the box, and put each "bar" (more like chocolate Capri Suns) in its own Ziploc. That way, I can spread them out between packages.