Hi grannie on ebay,
Thanks for contributing to our forum. Assuming you scheduled this listing using the "Edit and Relist" option on your Closed Listings page (which would be required to edit your shipping costs), it would only be possible to edit the post date and time from the Scheduled Listings page within your account.
Therefore, in order to ensure your shipping costs are updated as desired when the listing is posted, you would need to cancel the instance of the listing that is currently scheduled using the "Cancel Posting" button on your Scheduled Listings page and then re-schedule the listing anew from your Closed Listings page.
On the other hand, if you were to schedule a relist using the “Bulk Relist” option on your Closed Listings page, using the “Edit Listing” option on your Scheduled Listings page would allow you to edit any of the same details you were able to edit when originally scheduling the relist.
Alternatively, you could potentially allow the listing to post in its current state and then update your shipping costs in the listing using the eBay Revise your Item form once it is active as LifeisLike mentioned above.
I hope this helps. If you need any further assistance with your account, please feel free to contact our Customer Support team by mousing over the “Help” tab and selecting the “File Support Case” option.
-Mike