This is a complicated question to answer without getting into the nitty gritty of the item(s) and weight and dimension. Plus, whether your combined postage discounts were flat-rate or calculated. Plus, different rates will apply depending on whether the item will be shipped domestically or internationally. Pus, the courier/method used. There are so many variables to shipping that it is easily the most complicated aspect to online sales. Most of us have taken months to get it to a fine art and still there are areas where oversights occur based on the limitations of the system available to us.
Ultimately, you cannot ask the buyer to pay anything additional if they have already paid for their item so your options are to ship and take the loss on postage or ask to cancel the order(s) and take the defect.
But with more details, others may be able to help you avoid this in future.
