I personally just found out about this. It's between the 'wish list' and 'reviews' buttons on product pages so I always ignored it, but it has completely changed my order process.
I ordered "Beast Kingdom Horny Tiger Hard" and I got it yesterday. I'm trying to leave my review (its not a bad way to spend an early morning I'd say), I even have photos this time to boost my explanation.
UPDATE: I figured it out. If you go to Purchased Items and look to the right, there will be a check box next to every item. It's much easier to find on PC than mobile.