Should I miss the opening band to buy my friend's concert shirt?

My friend handed me her cash as the lights dimmed. 'Medium black shirt. I'll save our spot near the stage,' she said. There were about thirty people between us and the merchandise table.

I'd sent her three songs by the opening band that morning and said I wanted to catch their set. She hadn't listened; she'd come for the headliner. I joined the queue with her while we still had time, but I hadn't volunteered to finish the purchase alone.

When I said I was going into the hall, she said the shirt might sell out. I'd bought my poster before she joined me, so she thought waiting for her was a small favour. From the queue, I could hear the first song starting.

I gave her the cash back, and we spent the opener standing in different places. Is there a way to say no to this sort of concert favour without turning the whole night into two people counting what they did for each other?