SQ purchase

You purchase through your app store's currency, at least that's how it works on android. It'll charge you through your Google Play account and you can use a credit card or gift card if you have a balance. I assume the Apple Store is similar.

I'm from Eu too and I can confirm that you can purchase quartz easily, just link your debit card to your google play account as payment method and you'll be fine, even better you can use your paypal (i used it on the previous gssr and i purchased my quartz easily and without risks)

EDIT: sorry I assumed you are playing on android and gave the answer with that in mind, idk how it works for apple devices

So I live in the uk where fate grand order isn't available through the Google store, however downloaded it through a different app. I've also got an android. When I have bought saint quartz before it has been done through the Google store with my account but it confirms if I want to pay however much in dollars. Unless you know the £/€ to $ ratio is, you'll only have a rough idea how much you spent in your currency until you check your bank statement.