EBay forced it on me over a year ago, even though I don't want immediate payment. I prefer buyers have the option of being able to combine postage. As already suggested, get your buyers to add the items to the cart and then request a total. Unfortunately, that doesn't work if they are using the app.
If they only have access to the app, suggest they use the desktop version through the browser on their phone/device (Chrome or Safari) and see if that works. If it doesn't, find out which items they want and revise one of the listings to include all the items they want. Don't forget to change the price and alter the weight and dimensions so the correct postage is calculated.
Instead of ending the listings of the items you are adding to a single listing, you can revise them to make a new listing, so you don't lose that listing for nothing.
