Hmm, when I use your setup instructions and place an order containing that product, instead of

Sub-Total: €29.90
Flat Rate (Best Way): €5.89
19%: €5.57
Total: €35.79

... I get

Sub-Total: €29.90
Flat Rate (Best Way): €4.95
19%: €5.57
Total: €34.86

Any idea what's different (other than the result)?