One Sport Technology allows you to add merchandise to your online registration form.
Quick Steps: Shop> Inventory Management > Products
Firstly you will have to create a Product Category. Once this has been created you can add in your merchandise items. For the detailed instructions on how to add a product please see the relevant help article here.
The setting you will have to ensure is turned on is within the OTHER menu. Please ensure that the APPLICATION DISPLAY is set to either BOTH or MEMBERS. 'Both' allows the item to display on your registration form as well as on your online store. 'Members' will only display the item on your registration form.