diff --git a/services/nginx/app/objects/orders_o.php b/services/nginx/app/objects/orders_o.php
index f37b49c8..a039d3dd 100644
--- a/services/nginx/app/objects/orders_o.php
+++ b/services/nginx/app/objects/orders_o.php
@@ -615,7 +615,12 @@ class orders_o extends db
     public function getOrderItems(int $order_id): array
     {
         global $db;
-        $sql = "SELECT * FROM order_items WHERE order_id = $order_id";
+        // Order primary items first (related_item_id IS NULL), then addons grouped by
+        // their parent (related_item_id ASC), and finally fall back to insertion order
+        // (id ASC). Without an explicit ORDER BY, MySQL is free to return rows in any
+        // order, which causes the FE tree-builder to render addons before their
+        // primary on the invoice and POS displays.
+        $sql = "SELECT * FROM order_items WHERE order_id = $order_id ORDER BY (related_item_id IS NULL) DESC, related_item_id ASC, id ASC";
         $result = $db->query($sql);
         $order_items = [];
         if ($result->num_rows > 0 && $result) {
diff --git a/services/nginx/app/tests/Unit/Orders/OrderItemsListingOrderingTest.php b/services/nginx/app/tests/Unit/Orders/OrderItemsListingOrderingTest.php
new file mode 100644
index 00000000..ec7d4f22
--- /dev/null
+++ b/services/nginx/app/tests/Unit/Orders/OrderItemsListingOrderingTest.php
@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+it('orders the SELECT in getOrderItems so primary items precede their addons', function (): void {
+    $ordersObjectFile = app_path('objects/orders_o.php');
+    $content = file_get_contents($ordersObjectFile);
+
+    expect($content)->not->toBeFalse();
+
+    // Locate the getOrderItems method body.
+    $start = strpos($content, 'public function getOrderItems(int $order_id): array');
+    expect($start)->not->toBeFalse();
+
+    // Bound the search so we don't accidentally match unrelated SQL further down
+    // the file (the helper around line 645 in orders_o.php also uses ORDER BY id DESC).
+    $end = strpos($content, "\n    }\n", $start);
+    expect($end)->not->toBeFalse();
+
+    $methodBody = substr($content, (int)$start, (int)$end - (int)$start);
+
+    // The SELECT against order_items must include an explicit ORDER BY so MySQL
+    // does not return rows in undefined order (which has been observed to put
+    // primary order items after their addons, breaking the FE tree-builder and
+    // the invoice line listing).
+    expect($methodBody)
+        ->toContain("FROM order_items WHERE order_id = \$order_id")
+        ->and($methodBody)
+        ->toContain('ORDER BY');
+
+    // The ORDER BY must place primary items first (related_item_id IS NULL DESC),
+    // group addons by their parent (related_item_id ASC), and fall back to
+    // insertion order (id ASC).
+    expect($methodBody)
+        ->toContain('(related_item_id IS NULL) DESC')
+        ->and($methodBody)
+        ->toContain('related_item_id ASC')
+        ->and($methodBody)
+        ->toContain('id ASC');
+});
+
+it('does not leave the legacy unordered SELECT in getOrderItems', function (): void {
+    $ordersObjectFile = app_path('objects/orders_o.php');
+    $content = file_get_contents($ordersObjectFile);
+
+    expect($content)->not->toBeFalse();
+
+    $start = strpos($content, 'public function getOrderItems(int $order_id): array');
+    $end = strpos($content, "\n    }\n", $start);
+
+    expect($start)->not->toBeFalse()
+        ->and($end)->not->toBeFalse();
+
+    $methodBody = substr($content, (int)$start, (int)$end - (int)$start);
+
+    // The buggy SQL must be gone: previously this returned rows in whatever
+    // order MySQL felt like, leading to addons being listed before their primary.
+    expect($methodBody)
+        ->not->toContain("FROM order_items WHERE order_id = \$order_id\";\n        \$result");
+});