The easiest way to fix this is by creating a field function:
​https://app.datatrics.com/project/field-functions
​
We have created an example for you which you can use:
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output(new Intl.NumberFormat('nl-NL', {style: 'currency', currency: 'EUR'}).format(data.price * 1.21));
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In order to use this within your touchpoint:
{{#fieldfunction price_vat price=item.price}}
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This will print out the price multiplied by 1.21 (so including 21% tax).

For more information about field functions, click on this link.