We were following an older couple down the highway when their tire went flat. We pulled over behind them and my husband changed the tire for them. They offered several times to pay us - but I told them no , maybe one day they would get a chance to help someone and just to PIF.
If I have lots of stuff in my buggy to check out, I often let someone go in front of me who only has a few items. I feel like a goofball making someone wait behind me when I am not in a rush.