[Fuzzwah]

.... and when the waiter accidentally presses that button and changes the card which your business lunch is being charged to?

[cloudwalking]

Maybe the button only works when the card is near the phone?

[Fuzzwah]

That would be a pretty neat solution, but still has a window where it could be an issue.

Perhaps do the card selection from the app? Or at least an option to require confirmation in the app?

[amscanne]

That's a neat solution to the security problem also.

You go to the app on your phone and select the card you want. The coin will now function as that card for the next 5 minutes only. Otherwise, the coin is nothing.

Now even if your wallet is stolen, without your phone (+ unlock code, etc.) they don't have any of your cards!