The Doctor did not often shop.
There were plenty of other interesting places to go, things to do, people to see. Like, the crosswords. He could easily do the crosswords and that would be miles more interesting than shop. Or flossing! Good for the teeth, very fun, better than shopping.
Still, right now, the Doctor was shopping. The fact was, if he correlated going out for lunch (A) to the number of disasters he and Jack had to face (B), he'd come to the conclusion that B was directly related to A, meaning that (B|A) could be drastically reduced if he instituted C, groceries.
So, (B|A)=(A-C), would mean that B would have a negative quantity of C. So the Doctor attached a Follow Me drone to carry as many groceries as he could afford.
"One credit for a Churro," the Doctor commented, biting on the sugary concoction. "I'm not sure if that's even how much it costs to make one."