Throw error if postage parameter for precompiled POST request incorrect

app/schema_validation/__init__.py

@@ -29,6 +29,13 @@ def validate(json_to_validate, schema):
             validate_email_address(instance)
         return True
 
+    @format_checker.checks('postage', raises=ValidationError)
+    def validate_schema_postage(instance):
+        if isinstance(instance, str):
+            if instance not in ["first", "second"]:
+                raise ValidationError("invalid. It must be either first or second.")
+        return True
+
     @format_checker.checks('datetime_within_next_day', raises=ValidationError)
     def validate_schema_date_with_hour(instance):
         if isinstance(instance, str):

app/v2/notifications/notification_schemas.py

@@ -240,7 +240,7 @@ post_precompiled_letter_request = {
     "properties": {
         "reference": {"type": "string"},
         "content": {"type": "string"},
-        "postage": {"type": "string"}
+        "postage": {"type": "string", "format": "postage"}
     },
     "required": ["reference", "content"],
     "additionalProperties": False

tests/app/v2/notifications/test_post_letter_notifications.py

@@ -511,3 +511,23 @@ def test_post_precompiled_letter_notification_returns_201(
 
     resp_json = json.loads(response.get_data(as_text=True))
     assert resp_json == {'id': str(notification.id), 'reference': 'letter-reference'}
+
+
+def test_post_letter_notification_throws_error_for_invalid_postage(client, notify_user, mocker):
+    sample_service = create_service(service_permissions=['letter', 'precompiled_letter'])
+    data = {
+        "reference": "letter-reference",
+        "content": "bGV0dGVyLWNvbnRlbnQ=",
+        "postage": "space unicorn"
+    }
+    auth_header = create_authorization_header(service_id=sample_service.id)
+    response = client.post(
+        path="v2/notifications/letter",
+        data=json.dumps(data),
+        headers=[('Content-Type', 'application/json'), auth_header])
+
+    assert response.status_code == 400, response.get_data(as_text=True)
+    resp_json = json.loads(response.get_data(as_text=True))
+    assert resp_json['errors'][0]['message'] == "postage invalid. It must be either first or second."
+
+    assert not Notification.query.first()