What best describes the function of the US Postal Inspection Service?

The function of the US Postal Inspection Service is primarily focused on the investigation of postal offenses and civil matters. This includes protecting the postal system from criminal activities such as mail fraud, theft, and other illegal practices that might jeopardize the integrity and security of postal services. The Postal Inspection Service plays a crucial role in upholding the law specific to mail and ensuring that the postal network operates within legal frameworks, thereby maintaining public trust in mail services.

On the other hand, overseeing postal delivery efficiency, managing customer service relations, and handling package deliveries are important aspects of the USPS operational framework, but they do not fall under the jurisdiction of the Postal Inspection Service. Their responsibilities are distinctly separate from investigation and enforcement, which is why the focus on investigating offenses and civil matters is the most accurate description of the US Postal Inspection Service's function.