The U.S. Postal Service’s outsourcing of stamp sales and other retail services traditionally offered by post offices to Staples has been a simmering wound with postal unions, with nationwide protests and calls for a boycott of the office-supply retailer.

Now, one of the biggest labor battles in recent years is headed to Washington, where the National Labor Relations Board will rule in August on whether the Staples deal violates the Postal Service’s collective bargaining agreement with the American Postal Workers Union.

USPS launched a pilot program with Staples in 2013 to offer counter services  in 82 stores. After the pilot ended last year, Staples became an approved shipper for the Postal Service. With both programs, Staples employees staff counters inside stores and offer a range of post office services. The shipping program is now operating in about 1,000 stores.

The cash-strapped Postal Service said the arrangement would help save on labor costs, the biggest expense on its balance sheet. But the deal was met with angry protests from the APWU, which represents about 200,000 employees, about half the postal workforce.

The union said the Postal Service violated its collective bargaining agreement by illegally subcontracting work to Staples without bargaining first with the union. APWU called last year for a boycott of Staples stores and the company’s Web site.

The average wage of a post office employee is $25 an hour. A sales associate at Staples makes about $8.50 an hour.

The union filed charges last fall with one of the NLRB’s regional offices, in Baltimore. The regional director ruled against the Postal Service in late June, finding that the union’s claims had merit and violated the National Labor Relations Act, according to the board. The Baltimore region director filed a complaint against USPS and ordered a hearing in August.

“This ruling represents an important step forward in the battle against the privatization of our nation’s public Postal Service,” APWU President Mark Dimondstein said in a statement.

“Every person who organized their friends and neighbors to boycott Staples and warned them about the dangers of allowing a private company to take over the mail helped us get to this point.”

Postal Service spokeswoman Darlene Casey said the agency could not comment since the issue is the subject of litigation.

USPS is required to file a response to the complaint by next week. An administrative law judge from the labor board is scheduled to hear the case at 10 a.m.  Aug. 17 in Washington.