Invoice calculations

For each item, line total = max(quantity, 0) × max(unit price, 0).

subtotal       = sum(line totals)
discount       = min(max(requested discount, 0), subtotal)
taxable amount = subtotal - discount
VAT            = taxable amount × bounded VAT rate / 100
grand total     = taxable amount + VAT
amount paid     = sum(confirmed payments only)
balance due     = max(grand total - amount paid, 0)

Money is rounded to two decimals at calculation boundaries. Recorded, Rejected, and Reversed payments are excluded. This is operational VAT calculation only—not tax filing or tax advice.