It is possible to import Sub Total Lines in Invoices / Estimates into QuickBooks Online using SaasAnt Transactions (Online) application.
Importing Sub Total Lines for Invoice/Estimate in QuickBooks Online
Kindly map one of your file column to the "Show Sub Total" field and fill with values like "TRUE", "T", "YES" or "Y".
The particular row which has "Show Sub Total" should have the information like the below image.
It will be reflected in QuickBooks as below,