To add Tickets from the Shopping Cart, simply click the Add button on whichever ticket you want to add or click Add All.
After you choose your Add option, the screen will refresh and the line will be added to the invoice. The Ticket numbers will be reflected in the description of the line item to help differentiate the line items on the invoice.
Removing a ticket line item will move it back to the Shopping Cart.
Once the ticket has been added to the Invoice, if you head back to the ticket and click the Add/View Charges button, the line items there will reflect that they were added to an invoice.
That's it for now :)