When order is downloaded from Shopify / Magento 2 / Prestashop / Woocommmerce in some cases those systems are returning empty tax. For example, if you go to the menu “e-Commerce Integration → External Order Data“ and look into details of the order, you will see that in specific sales order line there is no tax (e.g. see screenshot below for example based on order downloaded from Magento 2)
Default connector behavior is to also leave tax empty also on Sales Order created in Odoo. But if you use Odoo for accounting in EU countries, you need to apply special “Reverse Charge“ tax to sales order line for proper accounting reports. For this there is special setting on “Sales Orders“ tab with name “Behavior on Empty Taxes“. You can select “Set Special Tax“ value there and in appeared dropdown select special Zero Tax that will be used in place of empty tax.
