Enable pledging in "preparing for shipping" stage

If you'd like to let backers add items to pledges after collecting their shipping address, you can do it in Project -> Settings -> Orders. Switch the toggle to "Yes" next to "Allow adding new items in 'Preparing for shipping' stage."

When you turn on this option, backers will only be able to add new items. They won't be able to remove already paid ones or change their shipping address.

Note: This will only work for orders in the "preparing for shipping" stage.