Thanks for the reply.
Yes, I think I will go to the library and log into Ebay and select the 'USPS with printer option', with the $3.84 estimate. Yes the shipping cost was $3.84 for the package that arrived at my place. I will return it in the same package.
I will see if I can just print off the label at the library, on a regular piece of paper, and then tape it securely to the package. I did some more research (I wish they would just explain it step by step after you click on 'return', but they don't) and found out that the printing of the 'return label' has something on it (a qr code?) that provides tracking info, which they also recommend.
When the post office scans something on the package I give them, the postage gets paid by ebay, and deducted from my refund.
Ebay has a deal with the USPS to get lower rates than I could get in person. The USPS told me I would pay $5.10 to send the package, including tracking. So Ebay's rate is less.
Thanks for the help. It's off to the library tomorrow!