Apparently United released the MileagePlus X app in the fall of 2014 but I am just finding out about it. With it, you can use your Sappire Preferred (or other MileagePlus linked credit card) or your United MileagePlus credit card to earn additional United miles.
The app allows you to purchase gift cards for Amazon.com, other online retailers, and local participating retailers to earn varying amounts of United miles. For instance, you can use the CSP to purchase an amazon gift card for 2x (+ 25% extra) united miles. If you were to buy a $ 100 amazon gift card, you would receive 225 United miles, plus the 100 UR points.