Calgary RCMP say they have stopped a sophisticated mail fraud scam that targeted people in the United States.

The police found thousands of letters asking the recipient to be a mystery shopper and check the customer service at a money transfer store.

They are sent a cheque to deposit and then withdraw funds to wire back.

When the banks discover the cheque is fake, the victim has to pay the money back.

Staff Sgt. Les Dolhun says these scams prey on people who are uninformed.

"Someone sends you a cheque out of the blue and says, 'Please cash this, keep some of it, send us a portion,' you should immediately be suspicious," he said. 

"The cheques are for a much larger denomination than they're asking you to retain. They're offering you to keep five or 10 per cent and send on $1,800 or some larger amount there's a pretty high risk that this isn't going to go well for you."

Ten people in the States lost $21,000 all together.

A man and woman from Calgary, and a man from Cochrane have been charged.