I discovered last week that USPS Tracking of your package is free even if you don't print a label from eBay or PayPal. If you prepare your own label and attach stamps for postage, you can take the package to a USPS office and they will add Tracking at no charge. This applies only to first class or Priority packages and parcels, not to first class letters. Tracking is also free if you buy the postage at the Post Office rather than using your own stamps.

I think it's also free with Parcel Select service but I haven't used that so can't say for sure.