To manually do it after purchase, but before buyer pays, click on Send Invoice on one of the items. All will show up to select and combine.
Otherwise, to combine in the cart or automatically, go to your Shipping Preferences and enable Combined Payments.
https://www.ebay.com/ship/prf
Further down you will see options to edit both Flat and Calculated shipping rules and/or create Promotional shipping rules.
Come back if you need help setting this up.
