The "advice to book a service" is to be used for every service the client has to pay for himself on site. This advice is given for free and does not generate any income since the client pays directly to the provider.
Therefore, the price you indicate for this service is purely indicative. You can also use this service type to recommend free activities like visiting the neighborhood. In that case, you just set the price to 0 and describe what the traveler is going to experience.
If you mainly post trips on a budget or backapcking trips where you give out a lot of tips, we recommend you add a "trip planning" service to get paid for your advice.