The Simple Invoice software supports discounts to individual items as well as to the entire invoice.
Use 'Add discount button' to open new window. There you can enter fixed amount or percentage value. This applies to the entire Invoice amount.
You can also select method to calculate dicsount. Before Tax method calculate discount from net amount. After Tax method calculate discount from gross amount. Discount can be shown in two ways. In items row:
Or in summary:
If you want show discount in summary go to Document Settings and check Discount to whole invoice in Summary option.
If you want you can apply discount to particular item, just click in Discount column and enter discount amount. Note that discount is always shown as amount (not percentage).