I often have to split shipments and send backorders and special orders at a later date, free of charge. Currently, when I am preparing to ship a backorder, I mannually send emails out to all the receipients of the incoming backorders and provide them with a "free shipping" coupon code, so they can purchase additional products to add to the shipment without incurring more shipping charges. My conversion rate is usually between 30-40%, and it helps offset the price of multiple shipments. Customers also love the opportunity to save.
It would be great it there was a notification template that would include personalized and pertinent information (name, order number, incoming product, etc.).
This feature could be built into shipstation or integrate with Mailchimp.