+=item recurring_billing
+
+A flag indicating whether this is a "recurring" transaction. Different
+processors interpret this differently, but the interface defines three kinds
+of payments:
+
+N: non-recurring; the customer is not expected to make another payment using
+the same card or bank account.
+
+F: first use; the customer has never used this card/account before but is
+expected to use it again.
+
+S: subsequent; the customer has used the card/account in the past.
+