While many competitions provide t-shirts bundled into their registration (see the related article "Collecting shirt size from athletes" for more on that), it is pretty common for organizers to also offer other merchandise or vendor offers to their athletes at an additional cost.
If you would like to sell additional merchandise at registration, you can do so from the "Registration" section of your competition's dashboard.
To sell additional merchandise:
Select "Registration", then "Retail".
Click on "+ Add a Sale Item".
This opens the form pictured above.
Specify the item's name, description, price, and how many of them you have available for sale, then click "Save".
Once a retail item is created, users will be able to purchase the item at registration checkout.