The scammer's goal is not only to get the yummy treats - but also to get real cash as change for the purchase.
It happened three times last month in Solano County - located between San Francisco and Sacramento - authorities say.
Each time, the person gave the Girl Scouts a counterfeit $100 bill for cookies.
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Fairfield police say that usually, the goal of the scammer is to get cash in return.
Investigators say it happened twice at Lowes parking lots, and the third time was in front of a Peet's Coffee.
"It's just heartbreaking. These are children, you know? We are taking advantage of our children. In both cases, the person came across extremely genuine, just wanting to support the Girl Scout troops," said Jennifer Brantley with the Fairfield Police.
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One of the troops was victimized last year as well.
Because counterfeit money was used, the Secret Service is also investigating.