To answer my own question since I bought one guitar from Japan finally, I paid :
- 653,29 € for the guitar + delivery ;
and then :
- 30,00 € for administrative customs post fees ;
- 136,67 € for VAT (21 %) import Belgium ;
- 00,00 € random fees ;
- 00,00 € others random fees ;
- 22,15 €...