To do this you must be using eBay on a desktop/laptop/browser version as the eBay mobile app currently doesn't offer this:-
1. Add the items you want into your basket.
2. Go to your basket
3. At the top, above the items, click “Request Total”.
4. You will then have click “Commit to Buy”. This will then send us a request to combine the shipping cost.
5. We will then send you a new invoice and shipping total which you can now make payment for.
Easy as that.