additional test for fees

exercises/structs/structs3.rs

@@ -78,5 +78,6 @@ mod tests {
         let package = Package::new(sender_country, recipient_country, 1500);
 
         assert_eq!(package.get_fees(cents_per_gram), 4500);
+        assert_eq!(package.get_fees(cents_per_gram*2), 9000);
     }
 }