How would I flash the inventory menu after adding a new item?
This is a bit advanced to set up, but the GuiDropDownBar component has a "HighlightForTime()" function, that brings the bar down for however long you specify
To access that in quest script you can do :
G.Inventory.Instance.GetComponent<GuiDropDownBar>().HighlightForTime(2);
You could add a public function to global script to call when you want to do this.
If you want to make it flash, you'll have to do some more custom code in global script, or add your own component to the gui that does it. (maybe just sets visible on/off on a timer repeatedly)