Create a new invoice and add discounts in any of the three discount
fields given. They go in order of precedence so discount 2 will be
calculated over discount 1 and discount 3 over the result of discount 2.
For example, let's divide by two on every discount:

Unit price: 600.00 -\>

> - Disc. 1 = 50% -\> Amount = 300.00
> - Disc. 2 = 50% -\> Amount = 150.00
> - Disc. 3 = 50% -\> Amount = 75.00

You can also use negative values to charge instead of discount:

Unit price: 600.00 -\>

> - Disc. 1 = 50% -\> Amount = 300.00
> - Disc. 2 = -5% -\> Amount = 315.00
