Hellobee Boards

I would probably put it in a ziploc bag and mail in a bubble mailer with a piece of cardboard in the mailer to keep it stiff. (That way, even if the leaves don't make it in tact, she won't have bits of leaf falling out onto her.) I'd also write a quick note explaining what it is and that you hope it makes it in one piece!

The adhesive spray you use on puzzels?!

If this is a really sensitive thing, juuuust in case it does fall apart, I'd stage a photo of you 'opening the package' on a generic surface. so that your SIL can gush about how nice it looked coming out of the mail.

My mother did this when my siblings wanted to send large construction paper monstrosities to my grandma and we knew they just weren't going to make it intact.

Slip it in a gallon size ziplock, then between cardboard