Right, thanks.

In case someone else is wondering like me: the first time you read the card, you need to do so with a normal QR scanner, not with your wallet's (or at least, my bluewallet wouldn't work). That's what I missed. Then, you will be redirect to a page where you have the actual QR code that you can use to fund the tip card.

Once the tip card is funded, you can read it with a wallet and it will recognize it as a valid LNURL withdraw.