Returns one invoice with its money and state worked out on read, in integer CENTS: subtotal_cents, tax_cents, total_cents, paid_cents, credited_cents, adjusted_cents, remaining_cents (copy remaining_cents, never compute it). state: draft, sent, partially_paid, overdue, written_off, paid or cancelled. Overdue means issued, still owed and due before today in the business time zone (sales_settings.timezone, unset = Asia/Singapore); days_overdue counts days there. activity lists the newest 50 events; viewed_count and last_viewed_at count every view. payer_confirmations lists the payer's "I've paid" reports. An unknown id refuses with NOT_FOUND: find it with invoice_list or invoice_search.