How do I amend an invoice to add P&P

[shhhh]

Item sold with free p&p. BUT buyer now wants item posting recorded..
How do I add p&p to an invoice.im not successful atm..

There should be a box that says something like sellers discounts or charges. Type it in the box, but don't click recalculate, click preview invoice before sending at the bttom of the page in blue, then you can see if it has added it or not.

Ah right...not been able to try it with this sale but will remember for next time.
Fingers crossed x

My prob is that I kept recalculating it..

Thanks x