Good writing! Everything is easy to understand, even though some things are a bit off. The meaning is pretty clear!
I'll correct the things that a native speaker would say differently.
I still remember that several weeks before ,I went to the supermarket .I waited for a long time in the checkout queue but the customer before me still didn't finish her paying .When I was wondering how many things she had bought on earth ,I noticed that she was struggling to open her wallet and get money out .Her hands were slightly trembling and she seemed quite faint . I guessed she might have issues with her joints .Apparently ,she needed help, but helping her directly seemed a little bit impolite.After all ,Some people are embarrassed by getting things they don't ask for . I needed to find a polite and considerate way to express my willingness to help without implying that she couldn't help herself .So, I just smiled and asked : ,"Do you need help ?" She seemed quite grateful and handed her wallet to me .
Most of us are willing to give a handsto those handicapped people, but they need and deserve our respect as well ,so asking them politely before helping them is the best way .