Optional Order Reprice at shipment time

 We are working with a client who has two types of price lists i.e ORDER BASED and SHIPMENT BASED ORDER based pricing is valid only for the new orders booked within effectivity dates of that price list

SHIPMENT based pricing is valid both for the new orders booked and the orders shipped with in the effectivity date of that price list.

i have read that work flow 'Line Flow - Generic, with Reprice at Fulfillment   reprices the order at shipment time and tested it also.it works fine.

How ever the question is how can we use this work flow optionally at the time of shipment to consider only the shipment based pricing!