UK Fedex Returned Goods Fee

Has anyone had a psa returned to the UK and received a bill from fedex for £21.00 as a ‘returned goods fee’?

From researching online they regularly do it and have no legal backing to be doing so. In this case I have no contract with fedex, and the return has arrived (fedex are not holding it) No Duty or VAT is due as its exempt.

At First I thought it was a scam letter, and I now see they do it hoping people may just pay it, It is regularly for £21.

I am armed with a response but thought id try any UK buyers here first, Ive not had it before on past returns.

I wouldn’t pay it and if they follow up say postage was paid in full prior to delivery.