Dominion Retail Electric (MA) overcharged sent additional demand notice 6 months after they were paid in full, Internet

several years ago I started to receive unsolicited correspondence from Dominion Retail promising to lower my outrageous electric bill (supplied by Massachusetts Nstar.) It was a no brainer; Dominion would bill Nstar directly, I pay Nstar, Nstar pays Dominion................stupid Me.

Almost a year after contracting with them, I realized there were no savings so i contacted Dominion to cancel my account. Six months after I cancelled, the past due notices started to arrive from Dominion. I contacted them and explained that I never paid them directly, they were reimbursed by Nstar, and I cancelled 6 months prior to this $129.37 bill. Even though Nstar was responsible for paying them and my billed with Nstar is always current, It was put on my credit report. 

Today, several years later, I received an offer from CBCS collection agency to settle with them for 25% less than my "past due bill." How can i owe Dominion money if I never paid them directly and they dealt directly with Nstar. i called Nstar after the bills started to arrive-they had not a clue as to what I was talking about.

I can't believe they get away with this and i'm sure I'm not the only consumer that was targeted. If you're smart, stay clear of this company!