Is it also too big? Simple Delivery is supposed to go up to 20kg. If so you can cancel/refund the transaction with the buyer's agreement via the dropdown menu arrow for the item on your orders list.
Then relist selecting "Oversize" so you can opt out of Simple Delivery to arrnge and pay for your own postage. Then the buyer can buy and pay again.
So during the listing process after selecting category and entering item title scroll down and click on the box under the heading Package Size. At the very bottom of the pop-up you should see: Oversized. Select that and you'll get: Domestic postage/ Simple Delivery is unavailable for this item. Then click on Add Primary Service so you will be able to buy your own postage.
It has to be done on a laptop or desktop rather than the app. If the option doesn't show up try changing sub-category or category. As you're arranging for your buyer to buy again the actual category is not so important, eg. furniture definitely offers the Oversize option so you could choose that.
