Taxes, discounts and currencies

Each line is quantity × unit price, minus its line discount. An invoice discount (a percentage, or a fixed amount shared across lines in proportion to their value) is applied before tax. Tax is calculated per line at that line's rate and rounded to the currency's smallest unit; the invoice shows a tax summary per rate.

Amounts are stored as whole numbers of the currency's smallest unit (cents, pence, yen), so there are no floating-point rounding errors. Each invoice has one currency; totals in different currencies are never added together in reports.