Calculate Invoice Payment

Calculate the payment details for an invoice.

    This endpoint calculates:
    - Total amount due on the invoice
    - Applied payments (existing + proposed)
    - Applied credits (existing + proposed credit memos)
    - Payment term discounts (based on payment dates) - ALL OR NOTHING: discount only applies if paying full invoice
    - Remaining balance due

    Supports payment terms with early payment discounts (all-or-nothing).