recalling a debt that went to a collection agency

When i moved into my new house, i purchased an xfinity plan online because google maps said there were no xfinity stores in my location. A few days later, i saw an xfinity store on my way home and stopped there to receive the box from them instead as the shipping wasn't going to arrive for a week or so. The employee in the store assured me that he would cancel the shipment and that everything was taken care of. a few months later i started noticing two account numbers charging me for monthly internet service. Apparently the employee in the store created a second account, so i was being charged for the box that never arrived and for the one i got in the store. 

I called and explained the employees mistake, the customer service representative realized the mistake and said they would close the account for me. On my next bill cycle i noticed there was no longer two accounts and i believed everything has been resolved. Now i am getting calls and texts from Sunrise debt solutions trying to collect a debt of $300 from unpaid monthly fees. Neither the employee in the store nor the customer service rep solved my issue and now it has escalated to a collection agency where it is effecting my credit score and my ability to purchase things using credit.  

I had already spent two hours today talking to a customer service rep and his supervisor, neither of which seemed helpful. I no longer want to spend time telling this story, nor talking to "service" reps. I need xfinity to recall this debt from the agency and instruct the agency to remove the negative marks from my credit report. I want in writing that xfinity acknowledges the double billing mistake and that they will recall the debt.