Once they have been created in your catalog, Fixed Price (Type = Sellable or Signup)* 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 at Site Settings > Registration > Tickets > Add-on Items.
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.)
*Because virtually all states regulate (or ban) online raffles (gambling), items with type = raffle cannot be offered online. If you are in compliance with your state's regulations, you can offer raffle items as add-on items by editing type = sellable and FMV = price.
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.
Go to Site Settings > Registration > Tickets > Add-on Items.
Note: Only items that already exist in your catalog can be added.