errr my first reaction to the question heading the OP is why would you want to tell your parents that anyway?
I think you shouldn't tell her. It's private and none of her business. If you put it away somewhere that is private to you, she has no business rummaging around there. If she does, well it's her problem, she was nosy. You don't owe her an explanation about it, you're 18.
I would try to buy it myself at that age, not online, because I'd want to avoid letting her know. It really is not a conversation I'd like to have with my mom, even now. *cringe*