Fixed discount code distributes incorrect amount to primary service when combined with bundle on same order

Objective

• Ensure the invoice total correctly reflects the full applied discount when a fixed discount code is combined with a bundle on the same order.
• A $3 shortfall on invoice #TCI3667 (Axium Colorado, 7428 Coral Ridge Dr, 7/1/26) confirmed the bug is live and affecting real orders.

Background

• Neil Chini (Axium/Colorado) identified a $3 discrepancy on invoice #TCI3667: the Military Discount ($30 fixed, applied to all 5 services) shows $102 in total savings rather than the expected $105.
• The order contains the VA Loan Complete bundle, which applies fixed per-service discounts to 3 of the 5 services (Radon, Sewer, Mold: -$25 each). The primary service (Home Inspection - VA Loan) receives a separate primary service bundle discount.
• Tracing the invoice line items: Radon (-$31), Sewer (-$31), Mold (-$31), WDI (-$6), and Home Inspection (-$3). The Military Discount's $6-per-service share is correctly applied to 4 of 5 services but produces only $3 on the Home Inspection — the primary service.
• The bug lives in calculateServicePrices.ts. For fixed discount codes, the function splits the total evenly (perService = round2(totalDiscount / eligibleServices.length)) and uses a "last eligible service gets the remainder" pattern to prevent cumulative rounding drift. However, the primary service also has a primaryDiscountAmount applied in an earlier pass, which modifies its preTaxAmount before the discount code pass runs. The combination of prior-pass price modification and the fixed-split rounding correction appears to produce an off-by-one allocation on the primary service.
• The result is a real invoice discrepancy: the client is billed $3 more than intended.

Product Decisions

Locked

1. Correct behavior — The full fixed discount amount ($30) must be distributed across all eligible services such that the invoice total reflects the full discount. No rounding residual should result in under-discounting any service.
2. Invoice accuracy is authoritative — The discrepancy is present in the invoice total ($1,083 instead of $1,080), not just the display; this is a billing accuracy bug, not a cosmetic one.

Open

1. Allocation method — The current even-split with last-service remainder correction is conceptually sound, but the interaction with pre-modified preTaxAmount on bundle+primary service combinations needs investigation. Should the fixed split use originalPreTaxAmount as the denominator base instead of the already-reduced preTaxAmount? Engineering should confirm whether the fix is isolated to the allocation order or requires a deeper change to how sequential discount passes interact.

Scope

Monorepo (attik) — Frontend

• Primary file: apps/frontend/src/util/functions/schedulingHelpers/calculateServicePrices.ts
• The fixed discount distribution block is the primary suspect. Focus on the interaction between the primaryDiscount pass (which modifies preTaxAmount on the primary service) and the subsequent fixed-code split, which uses preTaxAmount as the basis for even distribution.
• The isLast remainder logic uses findLastIndex over processedServices — verify that index is resolving correctly when the primary service is the first in the array and the last eligible service has already absorbed prior discount passes.
• Related: apps/frontend/src/components/scheduling/DiscountModel.tsx — UI for discount code application; not the source of the bug but relevant for confirming which service IDs are passed as eligible.

References

• Invoice screenshot: #TCI3667 (attached)
• Reported by: Neil Chini, Axium/Colorado (neil@axiuminspections.com), July 1, 2026
• Related: ATT-1923 (discount strikethrough display on primary lines — different symptom, same pricing surface)
• Codebase entry point: apps/frontend/src/util/functions/schedulingHelpers/calculateServicePrices.ts

Repro Steps

1. Create an order with 5 services: one primary service (e.g. Home Inspection - VA Loan) + 4 add-ons.
2. Apply a bundle that discounts 3 of the 5 services (not the primary) with fixed per-service amounts, plus a separate primary service discount.
3. Apply a fixed-amount discount code (e.g. $30) scoped to all services.
4. View the invoice and compare total discount savings to expected (bundle total + code total).
5. Confirm whether the primary service line receives the correct pro-rata share of the code discount.