After the checkout is finished, the session variable containing the shopping cart should be reset. The ShopSingletonParagraphTemplateModel.resetShoppingCart() method is currently called from an ftl template. Besides the fact that the UI should contain close to zero business logic, it has more disadvantages:

• what if the user closes the browser early? (template might still be rendered?)
• what if a rendering error occurs?
• what if the template gets modified, exchanged and the call is forgotten?
• what if some other templating technology is used?

The proper place to reset the shopping cart is probably in the processor chain itself (as last task).