First, make sure your merchandise is available in the Store > Merchandise subtab.
You can link merchandise to an activity in the Activity > Edit subtab.
Open the edit window of an activity and scroll to the Merchandise section under 'Additional Settings'.
In the Merchandise section in the Activity edition check the box for “This activity has the following linked merchandise”.
Select which merchandise is prompted to clients when they add the activity to their cart. Merchandise is prompted regardless if it was purchased it in a previous store session.
