I found this procedure for cancelling a label:

Here’s how to void a shipping label -

Log in to your eBay account.

Click My eBay near the top of the page.

Click Sold on the left side of the page.

Click More actions.

Click View order details.

Click Void shipping label near the bottom of the Order details page.

Select your reason for voiding the label.

Enter a message to the buyer, and then click Void shipping label.

We send you a "Shipment void confirmation" email after you void the shipping label.

( It's from 2013, but may still be useful)