UPS Charges on import after a significant delay

Storeroom5862

I challenged the import charges I paid on a motorcycle helmet and intercom system because retailers in the UK don't charge VAT when purchased together as it is classed as PPE. I emailed this to UPS who handled my shipment and they have responded saying they noticed that charges will have to be raised because the incorrect currency was used when it was declared. Is this my responsibility? The package was delivered back in April and I paid import charges and handling costs at the time, I was trying to claim back the VAT as it's exempt under these conditions but instead they've suggested they're going to bill me more? Any information or useful resources? UPS & Customs is a nightmare. Thanks.