Hide Postage setting when letters are not enabled for service

We don't show the sender addresses and letter branding on the settings page
if a service doesn't have letters enabled, so we should also hide the
postage for services that can't send letters.

(Postage can only be seen by Platform Admin users at the moment, but this will
change later).

tests/app/main/views/test_service_settings.py

@@ -89,7 +89,6 @@ def mock_get_service_settings_page_common(
 
         'Label Value Action',
         'Send letters Off Change',
-        'Postage Second class only Change',
 
         'Label Value Action',
         'Organisation Org 1 Change',
@@ -216,6 +215,25 @@ def test_should_show_overview_for_service_with_more_things_set(
         assert row == " ".join(page.find_all('tr')[index + 1].text.split())
 
 
+def test_if_cant_send_letters_then_cant_see_postage(
+    logged_in_platform_admin_client,
+    service_one,
+    single_reply_to_email_address,
+    single_letter_contact_block,
+    mock_get_service_organisation,
+    single_sms_sender,
+    mock_get_service_settings_page_common,
+):
+    response = logged_in_platform_admin_client.get(url_for('main.service_settings', service_id=SERVICE_ONE_ID))
+    page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
+
+    letter_table = page.find_all('table')[3]
+    rows = letter_table.find_all('tr')
+
+    assert len(rows) == 2
+    assert 'Postage' not in letter_table
+
+
 def test_if_cant_send_letters_then_cant_see_letter_contact_block(
         client_request,
         service_one,