eBay's Simple Delivery label cannot be cancelled. You'd have to cancel the transaction and relist for the buyer to buy again. You can opt out of SD during the listing process. Select Custom Postage under the Delivery Details heading during the listing or revising process. If you're not seeing that click on "Switch to advanced options" at the bottom.
If you decide to do that obviously discuss with buyer. Then click on the dropdown menu arrow alongside the item on your orders page and select Cancel order > Buyer asked to cancel. If they've paid it will be refunded in the process.