Letter have multliple rates, the agregate number was not taking that into account for the usage page.

tests/app/main/views/test_dashboard.py

@@ -777,11 +777,12 @@ def test_usage_page_with_letters(
     assert 'April' in table
     assert 'February' in table
     assert 'March' in table
-    assert '£18.94' in table
+    assert '£20.59' in table
     assert '140 free text messages' in table
     assert '£20.30' in table
     assert '1,230 text messages at 1.65p' in table
     assert '10 letters at 31p' in table
+    assert '5 letters at 33p' in table
 
 
 def test_usage_page_with_year_argument(

tests/conftest.py

@@ -2288,6 +2288,14 @@ def mock_get_billable_units(mocker):
                 'notification_type': 'letter',
                 'rate': 0.31,
                 'billing_units': 10
+            },
+            {
+                'month': 'February',
+                'international': False,
+                'rate_multiplier': 1,
+                'notification_type': 'letter',
+                'rate': 0.33,
+                'billing_units': 5
             }
         ]