Whether or not a service can be purchased when a guest books their stay depends on the type of service it is.
Your stay service (and attached products) can be purchased when a reservation is made, while all other services and products cannot; instead, they have to be purchased on site and manually added to the customer's bill in Commander.
Services and products
Services and products are always interconnected; you cannot create a product without first creating the service that it's attached to. This allows you set up a pricing system that takes both the price of the product and price of the service into account.
For example: if you set up room service with a price of $5, every separate order will have a 5-dollar service charge on top of the price of the products (no matter which products are ordered).