diff --git a/src/crates/monolith-commerce/src/shipping.rs b/src/crates/monolith-commerce/src/shipping.rs
index 989d567..7c850cb 100644
--- a/src/crates/monolith-commerce/src/shipping.rs
+++ b/src/crates/monolith-commerce/src/shipping.rs
@@ -170,7 +170,7 @@ pub fn calculate_rates(
                 length_mm: item.length_mm,
                 width_mm: item.width_mm,
                 height_mm: item.height_mm,
-                volume_mm3: item.length_mm as i64 * item.width_mm as i64 * item.height_mm as i64,
+                volume_mm3: volume_mm3_saturating(item.length_mm, item.width_mm, item.height_mm),
             });
         }
     }
@@ -287,7 +287,7 @@ fn calculate_rate_for_method(
                 .templates
                 .iter()
                 .max_by_key(|t| {
-                    t.max_length_mm as i64 * t.max_width_mm as i64 * t.max_height_mm as i64
+                    volume_mm3_saturating(t.max_length_mm, t.max_width_mm, t.max_height_mm)
                 })
                 .unwrap(); // safe: checked !is_empty() above
             for unit in units {
@@ -398,7 +398,7 @@ fn pack_into_templates(
     // Check each individual item fits in at least one template
     let largest_template = templates
         .iter()
-        .max_by_key(|t| t.max_length_mm as i64 * t.max_width_mm as i64 * t.max_height_mm as i64)
+        .max_by_key(|t| volume_mm3_saturating(t.max_length_mm, t.max_width_mm, t.max_height_mm))
         .unwrap();
 
     for unit in units {
@@ -450,9 +450,11 @@ fn pack_into_templates(
     let mut result = Vec::new();
 
     for parcel in &parcels {
-        let vol_weight =
-            (parcel.max_length as i64 * parcel.max_width as i64 * parcel.stacked_height as i64)
-                / divisor as i64;
+        let vol_weight = volume_mm3_saturating(
+            parcel.max_length,
+            parcel.max_width,
+            parcel.stacked_height,
+        ) / divisor as i64;
         let chargeable = parcel.total_weight.max(vol_weight as i32);
 
         // Parcel was built with stacking: max_length × max_width footprint, stacked_height.
@@ -553,8 +555,8 @@ fn parcel_can_accept(
     let new_width = parcel.max_width.max(unit.width_mm);
     let new_height = parcel.stacked_height.saturating_add(unit.height_mm);
 
-    let vol_weight =
-        (new_length as i64 * new_width as i64 * new_height as i64) / vol_divisor.max(1) as i64;
+    let vol_weight = volume_mm3_saturating(new_length, new_width, new_height)
+        / vol_divisor.max(1) as i64;
     let chargeable = new_weight.max(vol_weight.min(i32::MAX as i64) as i32);
 
     new_length <= max_template.max_length_mm
@@ -629,6 +631,17 @@ fn format_price(pence: i64) -> String {
     format!("£{:.2}", pence as f64 / 100.0)
 }
 
+/// Multiply three i32 dimensions into an i64 volume, saturating at i64::MAX
+/// instead of wrapping. Three i32::MAX values multiplied as i64 would otherwise
+/// overflow silently. Saturated values preserve correct ordering for max_by_key
+/// / sort and force oversized items to fail downstream fit checks rather than
+/// pack into a parcel with a wrapped (often negative) volume.
+fn volume_mm3_saturating(length: i32, width: i32, height: i32) -> i64 {
+    (length as i64)
+        .saturating_mul(width as i64)
+        .saturating_mul(height as i64)
+}
+
 // ── Tests ───────────────────────────────────────────────────────────────────
 
 #[cfg(test)]
@@ -670,7 +683,7 @@ mod tests {
             length_mm: l,
             width_mm: w,
             height_mm: h,
-            volume_mm3: l as i64 * w as i64 * h as i64,
+            volume_mm3: volume_mm3_saturating(l, w, h),
         }
     }
 
@@ -785,4 +798,29 @@ mod tests {
         assert!(!surcharge_condition_met(&s, Some("SW1A 1AA"), &rate));
         assert!(!surcharge_condition_met(&s, None, &rate));
     }
+
+    #[test]
+    fn volume_mm3_saturating_caps_at_i64_max() {
+        // Three i32::MAX values would mathematically be ~9.3e27, far past
+        // i64::MAX (~9.2e18). Saturating must clamp, not wrap.
+        let v = volume_mm3_saturating(i32::MAX, i32::MAX, i32::MAX);
+        assert_eq!(v, i64::MAX, "saturating mul should clamp at i64::MAX");
+        assert!(v > 0, "must NOT be a wrapped negative value");
+    }
+
+    #[test]
+    fn volume_mm3_saturating_normal_case() {
+        // Normal cart-item dimensions multiply exactly.
+        assert_eq!(volume_mm3_saturating(100, 200, 300), 6_000_000);
+    }
+
+    #[test]
+    fn oversized_dimensions_packing_rejected() {
+        // i32::MAX dimensions should fail to fit any sane template, not panic
+        // or pack with a wrapped negative volume.
+        let templates = royal_mail_templates();
+        let units = vec![test_item(i32::MAX, i32::MAX, i32::MAX, 1)];
+        let result = pack_into_templates(&units, &templates, 5000, &[]);
+        assert!(result.is_err(), "oversized item must be rejected, not packed");
+    }
 }