The truth of the matter is this: she's been stealing food. She's never worked a real job before, because she hasn't lived in a real society at any point in her young-adult life. The brief couple of years she spent in the real world weren't enough to prepare her; they were so short they may as well not have happened at all. She only knows how to fight, to steal, to survive in an apocalyptic wasteland. Not how to work retail.

And she's hungry. Her metabolism works faster than a normal person's; her healing factor ramps it up ten times over. She's constantly burning calories whether she wants to or not, so her stomach is a bottomless pit that free supermarket samples and stolen jars of peanut butter don't really cover.

This is why she hesitates, and this is why she ultimately says:

"Okay," and then, after a beat, "But only if you eat them with me."

She'd feel guilty taking his food otherwise. It's less bad, less strange, if it's having a meal together. Right?