@katarzynamos: Opera Cashback doesn't work, never got any cashback from you, even though:

1. My history of visiting online stores was saved (but the purchases were not)
2. Cashback was enabled accordingly to Opera instructions (before adding products to the cart)
3. Selected products were from categories that qualify for cashback

It is also not clear whether to use the built-in (dify) cashback extension or to install this external extension? I've tried both and none worked: with installed extension the built-in dify cashback can't be enabled and doesn't see the cashback. When the cashback is activated in the built-in dify extension ,then the installed Opera extension is disabled by it.
Yet none could bring me any money back to my account.

Why should we install an extension then, when it's built-in? What option should we use then? Should cashbacks in Opera settings be enabled or disabled, when using this external extension?
Can Opera stuff finally explain this?

@hucker: I agree, I'm from Poland and I also hate the inconsistency with commas and dots, especially in spreadsheets, where it makes a huge difference, and a spreadsheet from abroad can contain errors due to that.
Anyway, that Opera Cashback seems like a scam, never got anything from them, even though my history of visiting online stores was saved, but the purchases were not.
Also should I use the built-in cashback feature, or this extension? The built-in feature disables the extension and vice versa. One way or another, it doesn't work.