Incorrectly being charged a Variable Closing Fee

I have an asin which is a poster. We have noticed that for the past 12 months or so amazon have been charging us a £1 variable closing fee for this ASIN and we have contacted seller support to get this resolved. Seller support told us that the fee category was showing as 'books' for the ASIN and so changed it to 'Office Products', however, I am still being charged the Variable Closing Fee for every unit sold.

I have tried contacting the managing director email but they just said there's no adjustment to be made and everything looks correct.

Any ideas on how to fix this?