Buyer Opened a Case For An Item Held In Customs - Please Advise

[fabthedigitalone]

Hello,

Just over a month ago I sold an item from the UK to an EU country. Fairly high priced item (£500), but today the buyer opened an "Item Not As Described" case. Within the message board of this case, they state that they have not received the item. (How can it not be described correctly if they haven't received it?)

I checked tracking and the item has been held in the destination country at customs. Royal Mail tracking states that delivery is pending, and the local tracking website just states that the item has been held for exactly one month. 

I presume the reason for this is that the buyer has not paid import duties/taxes. (But currently this is just speculation)

Would anyone be able to advise on the best way to approach this situation?

Thank you.