Ebay can send a request to a seller to issue a combined invoice, IF a buyer is not using an app, adds all the items they want to their ebay cart, goes into the cart, clicks on "request total from seller" and then clicks on 'commit to buy'.

Ebay could  update their app to enable buyers to receive the same options available to desktop device users, but so far they have ignored feedback/suggestions/complaints about this anomaly.