You don't think that if the transaction settlement method cost a tenner to settle a six euro item, you could buy it with your special card that uses this settle

You don't think that if the transaction settlement method cost a tenner to settle a six euro item, you could buy it with your special card that uses this settlement method for six euros? If you use bitcoin, someone somewhere is moving those funds even if you are just "using your card". You're telling the payment provider to initiate a transfer to the merchant. How did you imagine this would work otherwise?