Good for you. There's stigma about this kind of thing among the immature, but if you're confident in yourself, there's no reason your girlfriend shouldn't have toys. It's normal and healthy. (Also, they can be a part of couple's play, too.)
I agree with the recommendations that you just wait and give it to her in person if you're concerned about the father. A plain unmarked package (which is what just about every retailer will use) will arouse his curiosity if he's that nosey. If you absolutely must send it, the Amazon suggestion is good, because then she can come up with a plausible lie.
If you have the choice, I recommend a sex-positive woman-owned shop like Toys In Babeland, which will have good reviews by real women on the site. You'll also have the satisfaction of supporting a nice independant retailer. I'm a cheap ass, but for this I wouldn't sweat a couple of dollars one way or the other.
Speaking of being a cheap ass: if you can afford it, the Hitachi Magic Wand is considered the best of the best, if you're feeling extravagant and generous. Otherwise, Google Toys In Babeland (I'm not looking up the site from my work computer here!) and read their reviews...