How do I say the cinema popcorn I bought is just for me?

As the trailers started, my friend reached across me and said, 'Pass the popcorn down.' There were four of us. I'd bought the smallest tub because that was what I wanted to eat.

At the counter, everyone chose their own snacks. She bought only a drink, apparently assuming we'd share mine. When I said I'd bought it for myself, she offered to send me a quarter of the price. I didn't want money; I wanted the portion I'd picked.

I passed it along rather than keep explaining in the dark. By the time it came back, there was a little left. Later she said she'd never thought of me as someone who guarded popcorn.

I don't want a snack to become a statement about my generosity. How do I make the arrangement clear next time without sounding as though every handful needs a bill?