3Wishes might have a good point. Are you sure the invoice is not in CHF?
Or better yet, don't look at Wizz's invoice. What price did you see on their website and how much was your credit card charged?
I remember an instance when I was flying home. I had the option of picking between my local currency and CHF. According to google, paying in CHF was better, since the it was about 10% less.
As others have said:
- Use Revolut. They charge you the number you see on google. In case there are any fees (if you exceed your free currency quota), it shows it separately, so you don't have to guess what happened.
- Let you credit card provider do the conversion. They almost always charge less than the merchant. If you ever pay in Europe with your Swiss card, you might get a prompt asking if you want to pay in EUR or CHF. Look at the rate and you will see the rate is higher than what Google says.
Additionally, I had a similar interesting experience when flying Wizzair, from Basel airport. I forgot to check-in online, so I had to do it at the counter. I was told I need to pay 37 CHF, and I could do it in Euro or CHF. I had a 50 EUR bill that I never used, so I thought I would use it now. I got 13 EUR change back.

Not sure if it's cause Wizz or Basel Airport's conversion rates. Perhaps I should have offered to pay in IRR.