Expect me to pay for their mistake!!

When i cancelled my account with vox I was told that I need to return the router. When Evette from the retensions department contacted me for them to arrange a collection time, she tells me that only the one router that takes the micro SD card needs to be returned. The router was returned and then I recieve an invoice stating that i owe money for the other router but Evette from from retensions department told me only the one router must be returned. Now vox expects me to pay for delivery of the other router because they made the mistake. This is not right, vox makes the mistake and expect me pay!!