Hello @valerie17
To remove the departure field, you need to add the trip duration in the pricing options of the trips that you have created. This will only set the start date and the departure date won’t appear in your site.
To set the trip duration, please go to, Admin Panel > Trips > Individual trip > Dates and Prices > Dates > Trip Duration
Regarding your query about the trip being disappeared, once the complete payment process is done, the booked trip will no longer remain in cart and to add the trips in the cart, you need to book the trip again. Also, if you are referring to the multiple checkout feature, currently the feature is not available.
Hope this helps,
Let us know if you have any query/confusion further,
Best Regards!!