I've already paid shipping... but the shipping company is asking me to pay more?

The UK is no longer in the EU so all sales are subject to taxes, duties, clearance fees at destination country.

• We no longer charge UK Sales Tax for orders from EU
• We expect the shipper may charge for the destination country Tax plus clearance charge.

"may" = value of order will affect the destination country's clearance process and the value of charges.

eg. Tax exemption on low value goods... but will end, see:

https://ec.europa.eu/taxation_customs/news/new-form-customs-declaration-low-value-consignments_en

https://www.avalara.com/vatlive/en/vat-news/eu-2021-ending--22-import-exemption--import-oss-returns.html

https://www.mic-cust.com/insights/news/detail/nd/the-eu-decided-for-the-abolishment-of-the-import-vat-exemption-limit-on-low-value-consignments/