The Charge Item report contains each of the charge items for the participants.
EG If a person registers for $50 and buys a T-Shirt for $30 then that is 2 charges for that person.
You can customise this report and then include only certain charge types - EG Merchandise only.
The report can be accessed by clicking "My Accounts" in the left menu, then clicking the "Reports" button for a particular event.
You can create a custom report by clicking the "Customise" button next to a certain report type.