Hello Stacie,
Welcome back to the QuickBooks Community! I'm glad you're enjoying this resource for working with QuickBooks Desktop. I'd be happy to go over the options for working with forms in the program.
I can see how having this extra field would be useful for the kinds of daily sales that your client is working with in order to be able to better track the taxable versus non taxable items. At this time, correcting the total HST in the subtotal area is the way to ensure that the amounts are correct for the sales tax remittances and other reporting since there isn't a way to edit the amounts line by line.
An option you can explore with tracking the sales tax amounts line by line, however, is to add a custom field to the form template. This custom field would need to be manually filled out, but it may be something that you can use and then customize reports to show this additional information. I can't promise that the custom field will show for all reports, but you're welcome to test it out to see if it works for you. Here are some articles that can help.
This is a great opportunity for feedback, which you can share by going to the in-product Help menu and choosing Send Feedback Online. By doing this, your comments are forwarded to the product development team for consideration in future updates.
Wishing you the best!