Invoice Template Generator

The engine sums each line as qty × unitPrice, applies a discount and tax, then computes balance due based on payment status. Late-fee preview uses the same simple-daily-interest formula as invoice-late-fee.

subtotal      = Σ qty × unitPrice
discount      = applyDiscount(subtotal, mode, value)
taxable       = subtotal − discount
tax           = taxable × rate                    // aggregate
total         = taxable + tax

balance       = paid='paid'    → 0
              = paid='partial' → max(0, total − amountPaid)
              = paid='unpaid'  → total

days_late     = max(0, asOf − dueDate − gracePeriod)
late_fee      = mode='flat'    → rate
              = mode='percent' → balance × (rate × 12 / 365) × days_late
                                 // monthly rate → annual → daily

The late-fee preview is informational only — it is never auto-added to the invoice total. Late-fee enforceability varies by state.