+C<billpkgnums>, C<setuprecurs>, C<amounts> are required and are parallel
+arrays. Each one indicates an amount of credit to be applied to either the
+setup or recur portion of a (non-tax) line item.
+
+C<custnum>, C<_date>, C<reasonnum>, and C<addlinfo> will be set on the
+credit before it's inserted.
+
+C<amount> is the total amount. If unspecified, the credit will be the sum
+of the per-line-item amounts and their tax adjustments.
+