When you mark an artwork as 'Available with delayed dispatch', the collectors will still be able to add it to cart. They see a notice of delay and if they're happy to wait, they can go ahead with the purchase.
To make an artwork 'Available with delayed dispatch':
Go to 'My Artworks'
Search / Find the artwork you'd like to edit
Click on 'More Actions' against the artwork
Click on 'Put on hold'
Select the date it can be dispatched on
Leave a note (optional) and save.