Markup on sales invoices?

In the invoice system is it possible for you to add an additional Colum that can be activated of required?

I sell products along with my services and like to add markup to products I routinly sell (depending on clients this % changes)
Is this some thing that could be implemented easily ? At the min I just use a calculator and input the future required however on big jobs this can be time consuming etc

Although the % figure would have to be not printable as to not disclose the mark up etc.

How many different products and how many different possible prices for each?

My first thought would be to simply have mutlipe sets of inventory items depending on the markup. No problem with a small product range but I can’t see if there is a way to set these up in a spreadsheet and import them.