Unable to customize invoices to add a column

One of the “Main Features” listed for this plugin is “Fully customizable HTML/CSS invoice templates”, but I’ve had 2 people try unsuccessfully. Currently there are 3 columns, Product, Quantity and Price. We’d like to have 4 columns by adding “Total”. Currently the “Price” column shows the total cost for that line rather than the unit cost. We need that to be the “Total” column, and change the “Price” column to the price per unit. For example, if someone buys 2 of Product X at a price of $15 each, currently is shows;
Product = X, Quantity = 2 and Price = $30
We want it to show;
Product = X, Quantity = 2, Price = $15 and Total = $30
The first person that tried said that any changes won’t be permanent because whenever we update the plugin the changes will be removed.
The second person said this can’t be done because these columns are from the main settings of plugins and we can’t change or add them.
Are they correct? If so, then I don’t understand what “Fully customizable HTML/CSS invoice templates” means.