I don't know if the cancellation will process. Hopefully someone else will be able to help with that, but for future reference don't cancel, just refund. How to do so depends on whether they have opened a request or not.
From eBay help pages:
Issuing a refund
You can issue a full or partial refund for items in Seller Hub - opens in new window or tab or My eBay - opens in new window or tab up to 90 days after the original transaction, provided there are no open cancellation or return requests, eBay Money Back Guarantee claims or payment disputes.
Here's how to issue a refund:
Find the order in Seller Hub - opens in new window or tab or My eBay Sold - opens in new window or tab.
From the drop-down menu, select Send refund.
Choose a reason for the refund.
Specify a refund amount for the item or for the order.
Enter the refund amount, and select Send refund.
You'll get an email confirming that the refund has been initiated, and another email once the refund is complete. The refunded amount will appear in your order details. You can refund up to 100% of the transaction.
If you're issuing a partial refund for an order, the amount will be split proportionally across each item in the order.
You may be eligible for fee credits if you issue a refund to a buyer on eBay without us stepping in.
If there's an open cancellation request, return request, report that an item hasn't arrived, eBay Money Back Guarantee case or payment dispute made outside of eBay against an order, the Send refund option won't be available. Instead, you'll have the option to send a refund when you respond to the request, report or dispute.
If there's an issue with the refund, you'll be prompted to use the Retry refund button. If you're still not able to issue a refund, you can change how the refund is funded or you can try issuing the refund from another device. We do not recommend issuing a refund outside of eBay, as this will not qualify for seller protections
