Consider tipping more generously. I normally don't tip anything more than rounding up to the nearest 10THB, but gas prices are up 50% in past year and meter rates have not increased at all. So drivers are making nothing these days if using meter.
All the "onward ticket" services do is book a ticket and cancel it within the 24 hr window that most airlines give you to cancel for full refund. You may as well just do that yourself.