Update:
I got tired of Hertz not responding, so I disputed a partial amount on my card.
I figured the difference using xe.com's historical currency conversion rates and disputed $5.80 of the transaction on my CSR. I selected "charged wrong amount" but there was a note that if it was currency conversion issue to please select "other". I selected "other" and it said there would be a chance to describe the reason for my dispute, but I was never given an opportunity to do so.
Immediately after I initiated the dispute Chase said:
As your credit card company, we value your business and want you to be completely satisfied with your credit card account.
We've credited $5.80 to your account. If applicable, we'll also credit to your account any interest charges related to the disputed transaction. These credits are permanent and will appear on your next billing statement.
So I guess that's that as far as I'm concerned. I wonder if Chase will go after Hertz for $5.80 or forget about it. The dispute shows up on my "manage dispute" screen but has no merchant listed (other disputes show the merchant), so I'm guessing they're just swallowing it. Agree with previous poster that I wish they would go after the unauthorized DCC merchants, but guess I'm glad my bank is taking care of me.