Single Tax Code mode will apply one tax code to all Sales, Orders, or Invoices associated to Shopify Orders which are downloaded to the point of sale. This setting is commonly used by users that require only one tax such as VAT or GST. When in this mode, Sales, Orders, or Invoices associated to Shopify Orders that do not have tax will be assigned the "No Tax" tax code in point of sale.
To setup Single/Default Tax Code mode, go to the Apps > Shopify > Taxes and select Single/Default Tax Mode and your Default Tax Code from the “Single Tax Code” dropdown. This list is populated from your Tax Codes in point of sale.
You must also be sure to create a Sales Tax, Tax Rate, or Tax Code in POS called "No Tax" with a rate of 0%.
In most cases, Users in countries that require Tax Inclusive Pricing will use this option. Please see Tax Inclusive Pricing for more info.