If you don't have a printer you can use your library's computer/printer facilities if it's open. If not you could ask a friend with a printer. They will have to log in as you so will need your password which you should change afterwards.
If that's not possible you will have to buy a postage label yourself at your post office and then contact ebay to ask them to change the tracking number on the return.
To contact ebay click on "eBay Help", top right of page, or on some pages it's "Help & Contact", top left . Scroll down to "Contact Us" and click on that. Select a topic, the nearest or any topic. On the next page, on the lefthand side you should get a link to "Have us call you" and "Chat with us".
