Follow the below steps and you will be able to allow combined shipping on eBay for buyers who haven’t paid for their items:
Go to Shipping Preference on eBay and look for Allow combined payments and shipping.
Click Edit on the same row.
Select the time, during which you are willing to offer combined payment and shipping to customers.
Hit Save to preserve your changes.
In case your buyers have already paid for their items, you can offer them a combined shipping discount via a partial refund. Make the changes on Orders (in the Seller Hub) or My eBay Sold.