Invoice total, with a discount and a tax rate

Type the line totals, the discount you agreed, and the tax rate that applies to you. It adds up. That is all it claims to do.

Line totals, one per line

Each number is one item you are charging for.

Discount, percent

Tax or VAT rate, percent

Type the rate that applies to you. Left at 0 it is simply not added.

Every line above comes out of the boxes you typed. Nothing is stored, nothing is sent, and there is no account behind this page.

It does not know your currency, so the answer is in whatever unit you typed.
Round-half-up and round-half-down differ by a cent now and then. A real invoice rounds per your own rule, and this page does not decide that for you.
This is arithmetic, not advice. What you charge and file is a question for your own rules, or an accountant.