FedEx invoice for delivery from Poland

[salj321]

Hi. I have received a invoice through the post from FedEx requesting an additional payment on some furniture bought on eBay and delivered from Poland to the uk.  I paid a delivery charge of £25 in the original transaction.  I am concerned that the invoice sent might be a scam.  Could anyone advise, please?  

[arkwebus]

... and a charge of £12 or so for FEDEX fees.  It's all to do with leaving the EU.

@salj321