I'm trying to mail this - will the chocolate hold up, or melt in transit?

I make it a rule not to mail baked goods during the warmer months. Melting chocolate is the least of your worries when it's warm. Anything with dairy can go rancid really quickly. Best to send overnight/next day delivery in a styrofoam pack with ice packs, if you really want to. And that can get expensive fast.

The melting point of chocolate is 86 degrees. That being said, you will want to be sure that your box isn't stuck in a hot truck or there's a heat wave in between you and your final destination. A couple things that may help are: Lining the box with styrofoam on all sides to keep out the heat. Or if you're a more advanced cook, tempering the chocolate before spreading. That's what chocolate companies do to keep the candy stable. Good luck. Your recipient is lucky to receive this!