DidI,
Teemu already contacted you. But the solution is simple here. You need to use dynamic taxation :
Go to Settings-Taxes and duties and scroll to the bottom: Enable dynamic taxation
It does not matter if prices are tax inclusive or tax exclusive in your catalog. This function will actually display the final tax inclusive price for your customer.
If prices are tax inclusive in your shop, the rule will work as follows:
20EUR - your country tax+ destination country tax
If prices are tax exclusive in your shop (as it seems to be the case) , the rule will work as follows:
20EUR + destination country tax