If you find that people are buying multiple items and sending payments separately (after all purchases have been made), they are most likely using one of the eBay mobile apps. I offer combined postage discounts, and have sent multiple invoices to buyers for combined post on iPhones etc, but the app doesn't recognise the invoices and just won't let them pay for all the items at once. (It can be done on a mobile device, but they have to go to 'Classic Site', not use the app).