How tax is calculated on an invoice

Tax is calculated per line:

1. Each line amount is quantity times unit price, rounded to the cent.
2. If the line is marked Taxable, the tax rate is applied to that line's amount and rounded to the cent.
3. The invoice's subtotal, tax and total are the sums of the lines.

Because each line is rounded on its own, a total can differ by a cent from taxing the subtotal in one go. This is expected.

The tax rate is saved on the invoice when it is created. Changing a tax rate later does not alter invoices or schedules that already used it.