The extent of government surveillance seems to know no bounds. First Edward Snowden shocked the nation with revelations about the incredible extent of government surveillance on average Americans’ cell phone and Internet data.

Now, from information buried in the United States Postal Service’s annual audit, we’ve learned that the USPS’ “mail covers” surveillance program has grown exponentially over the past two years.

The mail covers program has been around for decades. Currently, the USPS uses a program that takes a photograph of the outside of every piece of mail sent within the country, capturing names and addresses. The program is used to process mail, but it’s also available to law enforcement agencies. On request, the agencies can collect images of mail sent and received by targets for investigation.

Last year, USPS approved nearly 50,000 requests — both internal and from state or federal law enforcement.

That’s a huge increase over past years. From 2001 to 2012, the USPS received some 100,000 monitoring requests — an average of about 8,000 per year.

What’s particularly galling about this news is that the program has little oversight. The program has been covered in secrecy, and certain cases appear to have been approved without good reasons.

One of the better-known instances was in 2011, when a Maricopa County, Ariz., supervisor named Mary Rose Wilcox discovered that the combative sheriff, Joe Arpaio, had been monitoring her mail.

Wilcox had criticized Arpaio for his activities against illegal immigrants, many of which have been legally challenged.

In response to criticisms of the program, the USPS has insisted that it authorizes the sharing of sender information with law enforcement agencies only under limited circumstances.

But the truth is that we have no way of knowing. The tracking doesn’t have to be reported to anyone, which makes this program ripe for abuse.

Considering the extent of government surveillance that Americans have learned about over the past couple of years, “just trust us” is an unacceptable response.

Citizen privacy and protection against these kinds of searches is a constitutional right, not an occasional privilege. Congress needs to act now.