Thanks for the nice extension.
The extension is set up to work just on the grandtotal, but in our installation the product subtotals are calculated wrong.
The differenc between right and wrong calcuation comes in our case to 40 Rappen with good value arround 1000 Swiss Francs.
Can somebody retrace and solve this?
The calculation becomes correct if we activate the "discount price" option.
Wrong calculation:

The correct calculation:

the setup to grandtotal:
