A podcast last month said convenience store markups are around 300% on drinks. That really shows how much we overpay for the grab-and-go habit. Planning ahead like your sister did is the only way to beat those prices.
Man, my buddy Chris learned this the hard way last summer. He spent like forty bucks at gas stations in one day just on drinks and chips, then realized he could've bought a whole case of water for that price. Makes you wonder why we ever thought convenience store stops were a good idea.