Shanghai's postal service has started to require customers who use their own stamps to send overseas packages from one of 12 assigned post offices as part of an effort to crack down on counterfeit postage, local media reported Wednesday.

The change comes after more than a dozen downtown post offices uncovered a series of cases involving fake stamps, costing the postal service hundreds of thousands of yuan, said Kang Xinhua, who is in charge of service quality inspection at Shanghai Post, which runs the local postal service.

In response, Shanghai Post, a subsidiary of China Post, has outfitted 12 branches with equipment designed to identify counterfeit stamps, Kang said. Eight of the 12 are the main post office branches in the city's suburban districts. The others are located downtown.

Shanghai Post allowed residents to paste postage on all their own packages until September 1, when the new rule took effect, according to a report in the Shanghai Evening Post. Customers can still send packages bound for overseas at any post office in the city as long as they buy postage on-site.

The postal service was alerted to the problem in March, when an experienced employee in Putuo district discovered several fake stamps on a package that a customer was trying to send overseas. After police were called, they found 18,000 yuan ($2,834) in fake stamps in the customer's bag, Wang said.

The problem has only gotten worse over the summer, especially in the Lujiazui Financial Zone in Pudong New Area, where dozens of counterfeit cases have been reported. "A man caught on-site told me that he acted as a middle man, using fake stamps that he bought on the Internet to send packages abroad for customers at below market rates," Kang told the Global Times.

The postal service confiscates fake stamps on sight, said an official surnamed Xia in charge of market management with the Shanghai Municipal Postal Administration. "We know about the situation and are working with police to investigate these cases," he told the Global Times.

Counterfeiting stamps in large amounts carries a maximum sentence of seven years in prison, Xia said.

A search for "discount stamps" on one of China's largest online retailers found 3,000 shops. The two best selling shops make about 2,000 deals every month on average, according to online sales records. When asked, the two shops said they could not ensure the authenticity of the stamps.

"We never sell discounted stamps so we urge people not buy or use these stamps," Kang said.