When you go to the Store & Forms / Forms tab, Admins can see if there are any pending orders associated with a form. These represent items that were placed in a shopping cart, but were never purchased and are not real orders. We chose to provide this information in case you wanted to email these users to encourage them to complete their purchase for a sale that is about to close.
There is not a way for an Admin to delete the pending orders.
When a user or contact logs back into their account the next time, the shopping cart will be updated. Items no longer available will be removed and any change to items (change in sales price, sales tax) will be updated.