Adding to a Custom Gig


I am hoping someone here knows an answer to this or can direct me… I have a custome gig and now need to ship the final product to the customer. How do I add the shipping charges to the order… I didnt know this would be part of the gig when we started so I couldnt factor it in and for future work, shipping would always be dependent on the weight of the box. How do you modify the gig price once it has been ordered?