A person in a group I will be traveling with to Greece in October says there’s a 25% tax on credit card purchases in Greece. I can’t find any confirmation so I thought I’d ask the people on this forum for help.
You can't find any confirmation because such a tax doesn't exist. I wonder if they were a bit confused with VAT taxes. These taxes are built into the price you see on the object you are purchasing and really don't have anything to do with whether you are paying cash or credit card.
Whether you pay by card or cash the two main standard VAT taxes in Greece are 24% or 13% depending on the products/services.
The prices displayed always include these taxes.