Hmm.. They are trying to help you... They are not grilling you. Don't be so defensive.
http://smallbusiness.chron.com/bill-...ncy-56231.html
Looks that after it goes to collection you do have the right to dispute.
Also google the topic. You'll get more info.
But you do owe 1/10th at least. The machine worked 3 out of 30 days. I don't see how a court would rule otherwise. It's always best to try to work it out with the vendor. I'd call them and offer to pay them 10% for the 3 days that it did work. It is in your interest to work it out with them too. If you end up taking a wrong turn during the collection it can bite you...
Also check your rental agreement. Look if it talks about mediation and what not. Honestly not sure if most do or don't but something to look for....