Meaning "when they can't afford not to shop sustainably." People will continue shopping frivolously until those actions have pushed them to HAVE to shop sustainably.
I'm trying to put this into easier terms to understand. Think of it like this: you may want to go out to a nice restaurant every meal every day, but it wouldn't be very sustainable. Meaning, you couldn't continue that for very long because you would likely run out of money. But, if you insisted on doing so, you'd eventually run out of money, so you would only start eating sustainably until you couldn't afford NOT to eat sustainably.
Does this help?