I think you should send her something that will make her period seem fun, such as a tampon case (like this http://www.tamponcase.com/cyn/index.html
). There are tutorials on the boards for this as well. Or you could make her a little card, welcoming her to the womanhood club! OR (sorry, i have lots of ideas for this!), you could send her a package with some tampons, pads, homemade cloth pads (with instructions on how to use them, of course), or the ultimate (in my opinion), a Diva Cup http://www.tamponcase.com/cyn/index.html
to show her all of her menstrual options. There is also a knitted uterus I've seen on the boards a couple of times....
Whatever you choose, it is super thoughtful of you to even be thinking about this. When I first started my period, my mom baked me a cake, and while I was embarassed at the time, I definitely appreciate it now.