Kindly review the articles below to set up a Tax in QuickBooks Online.
http://www.intuit.com.au/r/product-updates/setting-up-gst-in-quickbooks-online

https://solarsys.sg/2014/09/10/quickbooks-how-does-the-gst-works-in-quickbooks

http://www.bookkeeping-essentials.com/canada-sales-tax-page1.html

If you are using U.S QuickBooks Online, please go through the below article for Sales Tax Import and you can skip this article.

https://saasant.freshdesk.com/support/solutions/articles/14000053541-import-transactions-with-sales-tax-for-u-s-quickbooks-online-companies

If you are using CA/UK/AU/IN (NON-U.S) QuickBooks Online, you may have different TAX Models such as GST, VAT, CST &, etc. But the basics of Tax Import is common across all versions of QuickBooks.

For simplicity, we can take “GST” as a tax model for further discussion. “GST” can be added in Sales and Purchase Transactions.

How to record Sales Tax and Purchase Tax in QuickBooks?

To record the Tax for a transaction, you have to do the following:

1. Tax Code should be given in Line Transaction (Product or Service Line Item or Account Line Item)
2. Need to mention Tax Calculation Models such as Exclusive, Inclusive or Not Applicable.

Example of Product or Service Line Item with Tax in QuickBooks

Example of Account Line Item with Tax in QuickBooks

QuickBooks needs the “Tax Code” to add taxes in your transactions. So “SaasAnt Transactions (Online)” does not ask you to provide TAX AMOUNT directly in the file. Instead, you will be asked to provide a “Tax Code” and our application will calculate the tax for you.

The tax information is given to "SaasAnt Transactions (Online)" by mapping relevant fields from the File. In the next section, we will see how the fields are mapped.

QuickBooks Field Mapping

Please review this link if you are new to the Field Mapping concept.

In the Field Mapping Screen, for each Transaction Type (Invoice, Sales Receipt, Estimate, Credit Memo, Expense, Check, CreditCard, Purchase Order), you will see the following QuickBooks Fields:

1. Line Item Tax Code or Expense Tax Code
2. Global Tax Calculation
3. Line Item Amount or Expense Line Amount

1) Line Item Tax Code (Application for Product Based Line Transaction) or Expense Tax Code (Applicable for Account-Based Line Transaction)

This field takes the PROPER Name of your Sales Tax or Purchase Tax. You can find the tax code name in QuickBooks UI. Tax codes can be found in QuickBooks by navigating to 

Sales Tax [OR] VAT [OR] GST (left navigation bar) > Rates & settings

Sample Image:

 You can also refer to the tax code in Sale or Purchase Transactions UI (QuickBooks). Check this Invoice Screen. The Sales Tax Code is "20.0 % S" in the example.

Sample Image:

2) Global Tax Calculation

This field defines the logic behind the tax calculation. It takes the following values only (Case Sensitive and No SPACE).

• TaxInclusive

• TaxExcluded

• NotApplicable

 TaxInclusive

The amount mentioned in “Line Item Amount” will be treated as Inclusive of Tax.

TaxExcluded

The amount mentioned in “Line Item Amount” will be treated as Exclusive of Tax. The tax amount will be added to the total amount.  

NotApplicable

The particular line transaction will be relieved from tax.

In QuickBooks: 

Workaround

If you don’t want to add a new column in your file for  “Global Tax Calculation”, there are two workarounds for you.

Workaround 1:

In the text box, you can map “Default Column 1” to “Global Tax Calculation” and provide default values such as TaxInclusive, TaxExcluded, or NotApplicable.

 Workaround 2:

 You can set the “Global Tax Calculation” in Import Settings also. Please check this link.

3) Line Item Amount / Expense Line Amount

This field records the cost associated with the Product / Service or Account. As we mentioned earlier, our app does not need the tax amount. Our app calculates the tax using the given Tax Code and Tax Calculation Model.