diff --git a/src/lib/pdf/format.ts b/src/lib/pdf/format.ts
index 1f49cf0..150783a 100644
--- a/src/lib/pdf/format.ts
+++ b/src/lib/pdf/format.ts
@@ -41,9 +41,18 @@ export function quoteSubtotal(items: { quantity: number; selling_price: number }
 
 /**
  * Sum of (order_qty ?? quantity) * cost_price across PO items. Differs
- * from `quoteSubtotal()` in using supplier-facing cost and order quantity.
- * A blank or zero order_qty falls back to quantity so it never zeroes a line.
- * Rounds to 2dp so the printed table doesn't show floating-point dust.
+ * from `quoteSubtotal()` in two ways:
+ *   1. Multiplies by `cost_price` (the supplier-facing price), not
+ *      `selling_price` (the customer-facing price).
+ *   2. Prefers `order_qty` when set, falls back to `quantity` so a
+ *      blank `order_qty` (the common case when the operator hasn't
+ *      adjusted the supplier-facing qty yet) never zeroes the line.
+ *      A literal `order_qty: 0` is also treated as missing — that's
+ *      almost always an operator mistake.
+ *
+ * Rounds to 2dp so the printed table doesn't show floating-point dust
+ * (1.0000000003 R etc). The supplier-facing PO uses this for both
+ * `Subtotal` and the base of the `VAT (15%)` line.
  */
 export function poSubtotal(
   items: { order_qty?: number | null; quantity: number; cost_price: number }[]
@@ -51,6 +60,9 @@ export function poSubtotal(
   return Math.round(
     items.reduce(
       (sum, it) =>
+        // `||` (not `??`) is intentional: we want `order_qty: 0` to fall through
+        // to `quantity`, since a literal 0 on a PO line is almost always an
+        // operator mistake (blank input) rather than a real "zero qty".
         sum +
         (Number(it.order_qty) || Number(it.quantity) || 0) *
           (Number(it.cost_price) || 0),