But if the time to drive across town is worth more than $5 in the $125 case, it's worth more than $5 in the $15 case, and forming that habit loses big. (Unless driving across town once allows you to save on more than one item, but that completely breaks the example.)

Other than cognitive cost, I don't see any reason to speak in terms of habits rather than case-by-case judgments here.

In the car case, you know the cost of walking away is very high; this screens off the informational value of the price.