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If an event is hosted in Prague (or anywhere else not in NL) you need to include a custom invoice. To do this, check the file /usr/local/share/php/Webshop/Invoice.class.php and edit the file /usr/local/share/php/Webshop/Order.php and add your respective product Ids to the method in question. So for Prague, the method HasCsirt needs to include the product_ids of products in Prague.
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