At the moment, the only negative/subtraction that we can do is a simple discount.
That's not very flexible.
With other invoicing systems, it's easy to enter an item with a negative value.
This enables us to do things like show the full price for something, and then give a line for each offer that's being discounted from that price.
Or, show the full price on one line, and subtract the 'Deposit paid' value on another line.
A simple discount section isn't very useful.
Please enable negative values to be used!