I had been wondering about this myself and I just discovered how to link your UR points to Amazon:
On the "Rewards Home" tab in the UR mall (the home page) there's a list of links under "Featured:" on the left side of the screen from your perspective. The third link down you'll see "Use Rewards at Amazon.com". Click that and follow the instructions. Worked like a charm for me and now I have the option to use either UR points or Discover points (depending on which payment method I select) towards the balance of whatever I'm purchasing.
