Can I use it without installing their software on my smartphone? Question is rhetorical - of course not, and your smartphone also needs to pass Google's or Apple's remote attestation schemes. Good riddance.
Is it really just PayPal left offering a sane online payment service?
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From https://support.wero-wallet.eu/hc/en-us/articles/25599074240...:
> It is not possible to use Wero via a web browser or on a computer.
Yes, you can. They have an app, but also integrations into bank apps.
That's interesting. In Belgium the pre-integration Payconiq could not do that.