If you want to give your client a discount, you can do this directly from the application. On the one hand, there is no need to calculate the discount manually; on the other hand, you can better highlight a promotional discount on the invoice.

All you have to do is check the ‘Discounts’ box immediately below the invoice fields:
– then briefly describe the discount (loyalty discount, for example)
– and put the percentage that is reduced from the total invoice amount

That’s all, do not forget to save. The discount will appear with minus on the invoice: