Offering merchandise products represents a huge benefit regarding your own sales and also, from a customer's perspective, your service quality. But how does a customer know that a merchandise product exists for a particular offer?
The merchandise product will be shown to the customer throughout the booking process for the respective offer. That way, your customer will definitely get in touch with the merchandise product somehow if you connected it properly.
To increase the chance of a customer noticing your merchandise product, make sure you mention it in the description of your offer.
Add-ons are not connected to resources. Add-ons can be connected to activities just for the purpose of displaying them as a cross-selling suggestion for customers, their availability is not dependent to the connected activity.