メールタイトルの修正 A4領収証

app/Email/Guests/ReceiptA4.php

@@ -24,7 +24,7 @@ class ReceiptA4 extends Guest
 
     public function getSubject(): string
     {
-        return '領収証の送付';
+        return sprintf('領収証の送付 %s', $this->order->receipt_no);
     }
 
     public function getGuestParams(): array