Overage surcharge and GST: the booking journal credits a CHARGE_ORG surcharge gross to PLATFORM_FEE while GST_PAYABLE is derived from the nominal price only

[teetangh]

Context

#1460 (fixing #1458 and Sentry FAMILIARISE_WEB-28) made the BOOKING ledger posting balance on the INVOICE rail when a programme sets overageSurchargeBps > 0: the surcharge is the one funding amount outside Payment.originalAmount, so createEarningsFromPayment now credits it to PLATFORM_FEE (it is a markup the platform charges the organisation for exceeding its own cap, not consultant income).

That credit is booked gross. Payment.taxAmount, which is what the posting credits to GST_PAYABLE, is computed on the nominal price and is not re-derived for an overage. Under the Principal model (ADR 26) the surcharge is consideration for the same taxable supply, so 18% of it belongs in GST_PAYABLE and on the organisation's tax invoice.

What to check and align

1. How lib/payments/billing/invoice-rollup.ts taxes an OVERAGE_INVOICE_ACCRUAL line: whether GST is applied on the line subtotal (then the invoice carries GST on the surcharge and the journal understates GST_PAYABLE by 18/118 or 18/100 of it, depending on whether the surcharge is tax-inclusive), or not at all (then the invoice is short).
2. Decide whether surchargePaise is tax-inclusive or tax-exclusive at computeOverageForBooking, and make the journal split it into PLATFORM_FEE + GST_PAYABLE accordingly, matching the invoice.
3. The nightly ledger reconciler should compare invoice tax totals against GST_PAYABLE credits per period so a drift of this class is caught.

No programme configures a surcharge today (both overageSurchargeBps writers default to 0), so this is dormant. It becomes launch-relevant the first time an operator sets a surcharge. CA question for the ADR 26 list: confirm the surcharge is part of the taxable value of the consultation supply rather than a separate service.

[teetangh]

Verdict 2026-09-05 (booking closure pass, dev 9dc665a). Confirmed and dormant (overageSurchargeBps defaults to 0/null on every writer). The code admits item 1 at earnings-service.ts:988-991; the invoice takes the opposite fork (invoice-rollup.ts:130-134,152 runs deriveGstBreakdown over a subtotal that includes the surcharge), so the org invoice charges 18% on the surcharge while the journal credits GST_PAYABLE nothing for it — a directional drift, not merely an open question. Item 3 does not exist: reconcile-ledgers.ts has INVOICE_TOTAL_MISMATCH (:583-612) but no invoice-tax-vs-GST_PAYABLE check. One CA question gates the fix: (a) the surcharge is part of the taxable value of the consultation supply (matches what the invoice already does; only the journal moves), or (b) a separate platform service with its own SAC (second line item, interacts with #1369). The reconciler check can ship independently of the answer.