<< No fees, just the conversion from euros to dollars at about the going exchange rate.>>

Are you sure about that? Paypal added 2.5 pct to my conversion as the card I have to Paypal has no foreign fees. And when I called to complain that the exchange rate was 2-3 pct higher than the official one, they admitted they do that and said they do it to all foreign transactions. And it is on their website, just hard to find. They convert it before sending it to your credit card, they send dollars to your credit card. Maybe you just say within a couple percent of the current bank rate is about the exchange rate, but I look more closely. It was exactly 2.5%, it says so on their website.

Here is the page for companies that accept Paypal on their website, which is what Renfe does:
https://www.paypal.com/us/webapps/mpp/merchant-fees

It says there is a 2.5% conversion fee, and they charged it to me, not Renfe.

I have my paypal connecting to a checking account as primary source of funds. There is no fee for that. I guess since you had PayPal bill your credit card, it added fees.

I rarely use my paypal account. It is for the occasional sale or purchase on eBay primarily.