Just came back from France & received an email from Sixt with an invoice showing--you guessed it--that "the cardholder chose to effectuate payment in USD" with a 3% fee. If I "chose" DCC, it was buried in a mountain of fine print on the rental agreement. I certainly didn't choose it knowingly or willingly.

Bottom line: beware, beware, beware. If you rent from Sixt, be certain to ask at pickup whether they're "offering" DCC (and if so, to opt out).