hidden credit card commission in spain

took the best advice and acquired a liverpool vitoria credit card for use abroad in spain but have been ripped off twice by retailers.When I ask do you charge commission on this transaction they answer no but when the print out is presented there is a 3% commission charged and say it is my card who is applying the charge.I tried to refuse the goods and not sign but was told I had to take the goods as the transaction had gone through,not knowig where I stood I signed.Liverpool Victoria say I consented to the rate and that is all they could say but told me to contact Visa direct.How do I get round this problem in future.