Add join to LetterRates for the billing query.

Fix the yearly totals to work for letters.

app/dao/notification_usage_dao.py

@@ -1,6 +1,6 @@
 from datetime import datetime, timedelta
 
-from sqlalchemy import Float, Integer
+from sqlalchemy import Float, Integer, and_
 from sqlalchemy import func, case, cast
 from sqlalchemy import literal_column
 
@@ -156,12 +156,18 @@ def billing_letter_data_per_month_query(service_id, start_date, end_date):
         NotificationHistory.international,
         NotificationHistory.notification_type,
         cast(LetterRate.rate, Float()).label('rate')
+    ).join(
+        LetterRate,
+        and_(NotificationHistory.created_at >= LetterRate.start_date,
+        (LetterRate.end_date == None) |  # noqa
+        (LetterRate.end_date > NotificationHistory.created_at))
     ).filter(
         *billing_data_filter(LETTER_TYPE, start_date, end_date, service_id),
         LetterRate.sheet_count == NotificationHistory.billable_units,
         LetterRate.crown == crown,
-        NotificationHistory.created_at.between(LetterRate.start_date, end_date),
-        LetterRate.post_class == 'second'
+        LetterRate.post_class == 'second',
+        NotificationHistory.created_at < end_date,
+
     ).group_by(
         NotificationHistory.notification_type,
         month,