How do shipping labels work?

I don’t really understand what shipping labels are or how they work. The reason I ask is someone told me if a customer ships me a package and includes and extra shipping label I could send it back to them for free. What does this mean and how does it work?

The customer can use a regular shipping label on which he puts his account number on it so that his account gets billed. Another way is used by companies that have shipping from somebody else back to them billed to the company account. UPS and Fedex will pre-print labels with the company’s address as the "to" address. Also, many companies use the on-line systems to print labels on regular paper that goes into a shipping envelope.

Every shipping label has a unique tracking number on it. This number helps route the package to the proper destination and lets the shipping companies verify that the proper shipping fees are paid.

I hope this helps.
Gary