Why do you need a sub-15-lb commuter? Not saying you shouldn't have one ... but why?
As a person with tens of thousands of commuter miles behind him, I want a commuter to be reliable and sturdy, capable of dealing with bad weather, and able to haul my gear.
Why ride a 15-lb bike with a full knapsack and rain cape?
Well ... because you want to, I guess ... which is fine.
But why only $500?
And where did the 15-pound number come from? You know that would be lighter than the bikes the pros ride in the Tour de France, right?
Also ... five minutes in bile lanes, five minutes on the road with traffic .... you could ride a 30-pound bike and it wouldn't be noticeably faster. A ten-minute commute? You won't even be warmed up when you finish.
Further ... what do you ride now? How much other riding do you do?
Can you do all your own maintenance? This mostly matters if you get a used bike, which might need work to be fully ridable after purchase. Because you are so close to work, you can get away with walking. I found as a car-free commuter, going to bike shops was impossible because I had to go to work every day. I needed at least two bikes and a third wheelset just to make sure I was always at work on time.
Would the bike see other uses?
I assume you have a totally secure place to store the bike when it is at work?
I assume you cycle enough to know that a 15-pound bike is a pretty rare animal, right?