The issue is due to Magento recalculating VAT when a different country is selected. To fix this, go to Stores > Configuration > Sales > Tax and ensure that Price Display Settings are set to Including Tax for both product and shipping prices. Then, check that your product's tax class is set correctly under Catalog > Products. Make sure your tax zones and rates for both countries are configured correctly under Stores > Tax Zones and Rates. If the problem persists, you may need to override VAT calculation logic in checkout, which may require custom development. Additionally, using tools like notes ai can help track and optimize your tax settings and calculations to avoid such issues.
