Firstly, you need to check your "site preferences" in your selling account to make sure you have allowed combined postage. Regardless of whether you have in the past, sometimes settings change for some unknown reason, so it pays to check that one first. Then instruct your Buyers to add things to the shopping cart before paying and request a total. If they are using a computer they should get a "request total" button located at the top right of the page. If they do not get it, they are probably trying to buy via their phone, and often they will not get the button. They will have to commit to buy the items, request the total, and then you will get an email with all the details and prices, and all you have to do it change the postage cost and send it back to them, and then they can pay the amended amount. I do it all the time with no problems. I have requested in my listings to add items to the cart and request a total if buying more than one item. The only time I have had problems is if the person is using a phone, as often the "request total" button will not appear for them.
