Re-Authorization for credit card to ship backorders

We are new to NS and need some feedback on the best way to process credit card authorizations for back orders.  We normal ship back orders after the original authorization has expried.  We know we have the option of going into the original cash sales order and checking the "Get Authorization" check box.  But it is time consuming to open each sales order to do this.  Also, we have found that it will only get the authorization for the entire dollar amount of the sales order not just for the remaining amount resulting in a higher rate of declines.  I'm