The flow is

- Place order

- Purchase gift cards

- Cancel order

Uber sees a line item for $XXX, at say Vons, and that's all they see. Generally food items are non refundable/returnable, so they have to eat the loss on canceled orders. It's actually a pretty solid scam, the issue is hiding it at scale.

Several metrics probably make them stand out:

- ratio of canceled vs fulfilled orders; frequency thereof

- ratio of average cost of canceled order to that of other drivers

- ratio of canceled orders on the consumer accounts vs population

Had they stayed small time, they probably would not have been caught

Edit: a word