1.  Create or open a Per Usage Billing Item in Configuration, Products and Billing Items
2.  Select an Item ID, Description, and applicable Dates
3.  Set the Unit of Measure, Quantity, and Unit Price
4.  Check the box to "Limit Charges to" and punch in the Maximum Charge amount
5.  Select either "Per Charge for Record (so 1:1 basis), or Payer (so multiple Charges can add up to the limit)
6.  Select to Limit Charges by Date if needed
7.  Choose whether to show a warning message, create the Charge up to the limit, or Do not allow the Charge to be created