[Feature] Add a way to have discounts

[Phelras]

It would be great if there was a way to add in a discount on an invoice. As it is when trying to add in a negative number for a discount it won't let you proceed if the number is less than 0, but the total updates correctly for the discount.

A way to add in a "product" as a discount as a percentage or as a sum or a way to provide a total invoice discount percentage would be nice.

[kittendevv]

I agree with you, discounts would be nice to have. If I have time I will work on adding this.

[ficematt]

I know there is better ways to handle the discounts, but if you could in the interim allow negative values in the invoice lines it would be helpful. They can show up on the invoice as a value in parenthesis.