If the license is NC, they can’t legally monetize it by demanding fees, that’s the whole point of the NC license. (I don’t remember where I found your file so I don’t know what license you put it under, but the other guy wrote that it was a NC license, so I’ll take his word for it.)

I would recommend registering your files with Copyright Office / Library of Congress immediately. I think I remember that your publication date was early August so you are still within the initial 90 day period. Copyright.gov has the forms and FAQs. The forms are self-explanatory.

It’s up to $15,000 of free money to you if you use the CCB, or $150,000 for statutory damages if you go with the regular federal court system – but you have to register the files timely for that.

DefCad has a long history of ripping off people’s designs in this manner. I remember people complaining about it regularly in /r/fosscad (RIP in pepperoni). Frankly I think it’s long beyond past due for someone to take them to task.