It's been a long known issue that you can't combine postage on the phone app. I think eBay is telling porkies that no-one has complained about it. If your buyers only have a phone, no computer, you could cancel the transactions, refund and then relist the items they bought as a bulk lot BIN, with a single postage cost.

If you choose to just refund the excess postage costs, you could refund minus the fees you get charged on the total postage cost.