The Quantity default value is “1” in VTiger. If a user wants to show the product in the quote and doesn’t want to add its price in the total, he needs to set the Quantity to “0”. To set the default value follow the steps mentioned below.
Adding items with Qty 0.0 in order to list Optional on Quotes
That way the Products price would be visible for the customer but it would not affect the quote’s total.
Open file: layouts\v7\modules\Inventory\partials\LineItemsContent.tpl
Change data validation engine for Quantity. Around row 135 you find this code.