Dish Network LLC Trying to make us pay for a receiver box they did not ask to have returned Englewood, Colorado

We switched our service from Dish Network to another service.  Consequently we received notification from Dish to return their equipment.  They sent two boxes along with a letter requesting that we ONLY send receiver boxes with serial numbers which matched the ones in the letter

We removed our receiver boxes and checked them against the serial numbers.  Only one of our boxes matched these numbers.  We sent back the one receiver box with those numbers along with the remotes and cables to the receiver box. 

We no longer have the box which Dish Network did not want.

They are now billing us for this box.  We have spoken with three people at their Colorado location.  They will do nothing but continue to send a bill for this box.  Had they simply asked for us to return all equipment belonging to Dish, we would have done so.  But, they made the distinction of listing the serial numbers of the box(es) they wanted back.  As one of the boxes did not have the numbers, we concluded that they did not want it returned.