Because I am dumb and just figured this out.
You can review each of your 5 products each month, for 10 points each (50 points total). 100 points = $10 to spend on the site.
$10 = a free Birchbox every 3rd month, or spend it on a specialty box.
Very cool on top of everything else they are offering. I liked this box so much that I went ahead and signed up for an annual subscription for $110. Using code join110annual gave me 220 additional points for upgrading. In combination with my 1 purchase and reviews from ONE Birchbox, I have $30 credit (and got a free box with the annual rate, as well). That means the next specialty boxes they offer, I can already order for free, or if there's a must have in one of my boxes, I've already got $30 to spend on it.
Best reward program options I've seen so far that are not driven purely by "refer a friend". I did all of this on my own and feel like I really got a deal on something I'm really enjoying. Birchbox has also been around for awhile with tons and tons of reviews, so I feel like this was a solid purchase.