I don't see how this argument invalidates the "freedom" of free will ...
Free Will: The power of acting without the constraint of necessity or fate; the ability to act at one's own discretion.
In the example provided the individual faced countless choices from buying candy on two days, buying coke on one day, saving their money until they could afford both, stealing one or both items, trading sexual favours with the clerk in exchange for both items, etc. and they ended up making the choice that seemed most rational to them. The fact that they had the ability to rationalize the decision and make the choice demonstrates that they had free will ...