Technically merchants are not supposed to add a surcharge for credit card processing unless its a non face-to-face transaction and a convenience fee
can be added. However merchants margin may necessitate adding a fee
and if fully disclosed than its up to the cardholder to decide whether or not to make the transaction.
DCC is different, its a deceitful practice with the end result cheating the customer out of their money. Its akin to banks’ charging fees on debit card transactions that result in a negative balance.