How is the VAT amount determined?

The VAT (Value Added Tax) amount is automatically calculated at the time of payment based on the standard VAT rate set by the supporter's country or region of residence.
Supporters do not need to calculate the VAT themselves.

For information on VAT rates, please refer to the official sources in your country or region.

• European Commission
  https://taxation-customs.ec.europa.eu/taxation/vat/vat-directive/vat-rates_en

Please also see the following article.
Who is subject to VAT (Value Added Tax) collection?