Wait. The law office told you that there would be no interest, and they had it on their notes, and they still charge you interest?? If it was me, I would call them back and try to push that issue maybe trying to talk to a supervisor.
Going forward, I was told by a credit lawyer that anytime payment arrangements are made with ANY company, you should ALWAYS get the arrangements in writing before you send them a dime. So what if it pisses them off… They are obligated to comply.