Of course, You can make free trips available to the customer in the WP Travel plugin. If you do not want to set the price to the particular trip then you can enter the price as ‘0’ in the trip setting and no price will be displayed for that trip.
However, if you want to display information like pricing not available for this trip at the checkout page by going to Admin Panel > Trips > Individual trip(edited) > Checkout as shown in the screenshot below:
The above options will be handy if you want to offer trips for free or trips with no price. You might also be interested to provide group discounts on certain trips to your customers.
If you have any confusion then please post them in our official support forum.