Charged for a returned puck

[Ticura]

The Royal Mail app shows that my sky stream puck was delivered (returned) on 23 June but I have a text from sky saying I have been charged for not returning it. 
The sky app shows "return in transit " then "we can see it's on its way back to us " .

Does anyone know why I am being charged for this? and who I should contact?

[PandJ2020]

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@Ticura wrote:

Does anyone know why I am being charged for this? 

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They can take several weeks to process - but it appears to have failed this time.

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and who I should contact?

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Sky?  They will request proof of posting.