totalDue is the grand total on the invoice: the subtotal (livestock), plus buyer fees, plus tax. Tally computes it for you — it's not entered by hand. What still has to be collected is the balance due, which is the total due minus what's been paid.
This is a read-only, calculated amount. It moves only when the subtotal, fees, or tax change — not when payments come in (those reduce the balance due).
Technical section
Data model, API, and internals — visible to developers.