Transportr is a non-profit Free Software project and has never received a lot of donations. If you still want to show your appreciation or make a financial contribution, there are some ways to do it:
Liberapay is a recurrent donations platform.
They help to fund the creators and projects you appreciate.
You can also send donations via Bitcoin to this address:
More options might be added later. There is an issue on GitHub where additional options are being discussed.