Once they have been created in your catalog, Fixed Price (Sellable, Signup, Raffle) items can be offered for sale during the ticket sales process to ticket buyers and non-ticket buyers.
Items can be flagged to offer during registration in the Ticket Sales Settings or when creating/editing items in the catalog.
Add-on items purchased during ticket sales will display on the Check-in screen for distribution at your event (whether the purchases display for the ticket buyer or their guest(s) depends on where the item is offered on the form.)
To flag add-on items from the catalog:
Go to Catalog > Add a new item or Edit an existing item > Item Visibility
In addition to specifying where you would like the item to appear on the registration form, you can also opt to display the item's description and allow users to specify the quantity they wish to purchase.
To flag add-on items from Ticket Sales settings:
Go to Admin> Site Settings > Ticket Sales > Add-on Items.
Note: Only items that already exist in your catalog can be added from the Ticket settings.