+A direct (Business::OnlinePayment) transaction will return nothing on success,
+or an error message on failure.
+
+A third-party transaction will return a hashref containing:
+
+- popup_url: the URL to which a browser should be redirected to complete
+ the transaction.
+- collectitems: an arrayref of name-value pairs to be posted to popup_url.
+- reference: a reference ID for the transaction, to show the customer.
+