I'll delay posting the detailed mechanics for how the Economics system would work until there is more information (e.g. Economics Whitepaper), but effectively the card would be nothing more than a hard copy wallet of your funds (could also be used for simple cold storage).  It wouldn't convert the funds to EUR or USD as the existing MC/Visa systems do today as that's where the fees are incurred due to using their network.  

Let's be honest. The reason most merchants accept crypto are usually 2 fold:  1: its new and cool and they want to look new and cool. 2: there is a lot of "money" wanting to be spent and they want to get their hands on some of it.  However, that's about the limit of their interaction in the space for most merchants. They fully understand the existing price volatility and thus their rational self-interests compel them not to hold it for longer than necessary and thus immediately convert it to their native fiat after the sale completes.

If, however, that price volatility were to be substantially negated as well as incentivizing them to receive balance increases (interest earnings) on their existing holdings without having to do any Po"X" type of work then they would probably be more than willing to hold Radix tokens.  Not to mention that if their computers are online most of the day verifying network txns that need verifying (with a minimal amount of impact to their computers since the consensus protocol is supremely efficient)  they would also receive earnings payments for their portion of work from the new supply generated.