I'd be curious to hear more specific info about this. I've rented a car twice overseas - in the Azores last year and in Sicily just a couple of weeks ago.
I don't recall being offered anything extra at pick-up in The Azores, but I was given the hard push at the counter in Sicily. They were even saying if the car was stolen, I would be liable. And I was renting with my company's corporate code, which is supposed to give me more coverage.
The "full" coverage plan they were offering would have doubled the price of the car's daily rate. After almost caving, I remembered these add-ons are just a cash grab and declined.
I should do some additional research before I ever rent internationally again, though. I do always pay for this kind of stuff with my Capital One Venture card, which also gives more coverage, but I honestly did not refresh my knowledge of the details before this trip.
Another lesson I learned was that these international rental car companies can charge your credit card in local currency, and then charge you ANOTHER 3.5-4% to convert that charge back to US $. The problem with this is your cc does this anyway, and Capital One (as well as many other cards) does it for FREE.
My final cc charge was a good bit higher than the invoice I was provided after return. There was no documentation of the currency fee on the receipt. When I called Avis customer service, a less than helpful woman said that's what it was (a currency conversion fee), but when I asked why it wasn't even documented on my receipt, she insisted is was a "local" fee and it was "in your original contract". I told her that wasn't a helpful answer, and she hung up on me.
However, when I researched this on the internet, I found that it is a corporate Avis policy, and some counters will charge you as much as 7% because they can also tack on a "service fee" in addition to the percentage. My charge wound up being $50+ on a $800 bill, which is about 6%. I looked in my contract, but it's not spelled out to this degree of detail (most rental contracts are vague, and no one at the counter mentioned it either).
You also actually have the ability to "turn this off" in the settings of the Avis app. It's buried in the section where your cc info is stored, and of course the default setting allows them to charge the extra $. You have to toggle the button "on" to "authorize" international counters to charge your card in US $.