If you want your customers to purchase merchandise that is mandatory for an activity you can link it directly to it and offer it during the checkout.
Start by creating the merchandise under Store > Merchandise.
Select an activity that you want to have a mandatory Merchandise.
In the Merchandise section in the Activity edition check the box for “This activity has the following linked merchandise”.
In the list check those that you want to be Mandatory or Suggested.
It is important to note that if a membership is offered during check-out it will overrule the merchandise. As such, keep in mind that if you have mandatory memberships and merchandise you need to select which one you want to prompt.