Minor tweaks in response to PR comments

In response to [^1] and [^2].

[^1]: https://github.com/alphagov/notifications-admin/pull/4229#discussion_r869181152
[^2]: https://github.com/alphagov/notifications-admin/pull/4229#discussion_r869186063

app/main/views/dashboard.py

@@ -423,6 +423,10 @@ def get_monthly_usage_breakdown_for_letters(monthly_letters):
         postage_order[get_monthly_usage_postage_description(row)], row['rate']
     )
 
+    # First sort letter rows by postage and then by rate, clumping "europe" and
+    # "rest-of-world" postage together as "international". Group the sorted rows
+    # together using the same fields - "group_key" is used for both operations.
+    # Note that "groupby" preserves the sort order in the groups it returns.
     rate_groups = groupby(sorted(monthly_letters, key=group_key), key=group_key)
 
     for _key, rate_group in rate_groups:

tests/app/main/views/test_dashboard.py

@@ -1079,9 +1079,10 @@ def test_usage_page_monthly_breakdown(
     assert '140 free text messages' in monthly_breakdown
     assert '960 text messages at 1.65p' in monthly_breakdown
     assert '33 text messages at 1.70p' in monthly_breakdown
-    assert '10 second class letters at 31p' in monthly_breakdown
     assert '5 first class letters at 33p' in monthly_breakdown
-    assert '10 international letters at 84p' in monthly_breakdown
+    assert '10 second class letters at 31p' in monthly_breakdown
+    assert '3 international letters at 55p' in monthly_breakdown
+    assert '7 international letters at 84p' in monthly_breakdown
 
     assert 'March' in monthly_breakdown
     assert '£20.91' in monthly_breakdown
@@ -1110,7 +1111,7 @@ def test_usage_page_monthly_breakdown_shows_months_so_far(
 
 
 @freeze_time("2012-03-31 12:12:12")
-def test_usage_page_letter_breakdown_ordering(
+def test_usage_page_letter_breakdown_ordered_by_postage_and_rate(
     client_request,
     service_one,
     mock_get_monthly_usage_for_service,
@@ -1123,7 +1124,8 @@ def test_usage_page_letter_breakdown_ordering(
 
     assert normalize_spaces(postage_details[3].text) == '5 first class letters at 33p'
     assert normalize_spaces(postage_details[4].text) == '10 second class letters at 31p'
-    assert normalize_spaces(postage_details[5].text) == '10 international letters at 84p'
+    assert normalize_spaces(postage_details[5].text) == '3 international letters at 55p'
+    assert normalize_spaces(postage_details[6].text) == '7 international letters at 84p'
 
 
 def test_usage_page_with_0_free_allowance(

tests/conftest.py

@@ -2421,7 +2421,7 @@ def mock_get_monthly_usage_for_service(mocker):
             {
                 'month': 'February',
                 'notification_type': 'letter',
-                'rate': 0.84,
+                'rate': 0.55,
                 'chargeable_units': 3,
                 'notifications_sent': 3,
                 'postage': 'europe',