I think it's great that you have two customers who want you to work on their Ra2 systems in their vacation homes, and it sounds like a fun opportunity to travel while working. I think it's reasonable to charge for all travel time, including the time spent on planes, as well as any overnight stays. You could also consider charging a per diem for food and lodging expenses. As for billing for travel time, it really depends on your own policies and the expectations of your clients. I would suggest being transparent with your clients about your billing practices upfront to avoid any confusion later on. And since you're traveling for work, why not treat yourself to a little luxury and opt for business class flights? I understand that it may not always be feasible, depending on the budget.