I literally showed them their server taking over a minute to respond to my request. I even showed how the delay increased proportionally to the message size... Clearly doing more processing; classic DoS vulnerability... Doesn't leave much to the imagination! But they said they would not pay me anything unless I actually proved that it scaled and caused disruption of their service!
It seemed like they were baiting me into incriminating myself for a crime that they wanted me to commit against them. It's not even the first time that I've been baited by a software company into committing a crime against themselves. I never took the bait though.