Among salesmen, as well as players on the street, a stack is a unit of $10,000.00. When something cost 3 stacks, it cost $30,000.
That customer bought 3 stacks worth of products or services, that customer when down for 3 stacks. If the customer bought $34, 800 worth of products then he went down for 3 stacks, 4 pounds, and 8 hundos.