How do I explain that running an errand doesn't mean I can carry chairs all evening?

My cousin saw my supermarket bag and said, "So you'll be fine helping set up the hall tomorrow."

I had already declined the chair-moving job because of my ongoing pain. The errand was something I chose and could leave halfway through. The hall setup starts at six, involves lifting and has a promised finish time. My cousin had taken me off the list, then put me back after seeing the bag.

I could help with ticket envelopes at home, but I am hesitant to offer. I worry that doing any part of the event will become another argument about why I refused the chairs.

I am following my own care plan and am not asking readers to decide what physical work I can do. I want to explain this particular difference without giving a pain score, an appointment history or a defence of buying groceries.

Would you restate the refusal alone, or offer the envelopes at the same time?