Provide fixed amount discounts in invoicing

Business users will now be able to provide fixed-amount discounts in invoices
This is applicable for both one-time as well as recurring invoices
This comes in addition to the percentage-based discounts, so users will now have an option to give 10% off or $100 off, for example, depending on the use cases.
Any tax calculation on individual items will be done after deducting the discount amounts, so the customer will not be paying any taxes on the discounts offered.