Q1. Where can I adjust the settings for a single invoice?
Click on the gear icon on the right side of the screen while creating an invoice. A menu should show up.
Q2. How to remove GST, item discount, or item quantity & unit from appearing in the invoice?
To remove GST, you can adjust the default GST settings to No GST.
To remove item quantity & unit, you can disable both options under Item Attributes.