Mrs Hartell said she turned around and saw a man looking at her, and tried to tell him thank you but not to worry about it and give the money back, but he walked away.
The man looked to be in his 50s and, from appearances, didn't seem to be very well off himself, she said.
She said the member of staff told her she had seen him do it before and he wouldn't take the money back, but the Chronicle was unable to track her down at Pak'n Save yesterday.
Mrs Hartell took to the Chronicle's Facebook in the hope he might see it.
She wanted to thank him and tell him it was people like him that made Wanganui a friendly and caring community.
Online she posted, "I have had a stressful two weeks. Last week I nearly lost my father and had to find money to get down to Wellington Hospital, and this week my mother in law is very ill in hospital. So this guy was an angel who not only helped me out, he made my day."
She kept chickens and thought if she could track the man down, maybe she could supply him with some free-range eggs.
Mrs Hartell said when she last checked Facebook after posting her story, there were three comments under it. The next morning, she was astonished to find that the post had reached across the world, attracting "likes" from England, America and Australia.
Commenters called for her to "pay it forward", as well as sharing their own personal experiences of showing, and being shown, similar kindnesses.
Others said it had restored their faith in humanity and spoke volumes about the goodness of people in Wanganui.
Mrs Hartell called the comments "just awesome", and said she would definitely be paying the favour forward.
One of the commenters suggested she ask Pak'n Save for their CCTV footage, which she thought she might look into.
Foodstuffs, which owns Pak'nSave said it had been in touch with the man, who did not want to be identified.
"He just likes the idea of doing something good, and it possibly coming back to him one day," spokeswoman Antoinette Shallue said.
He wanted people to know there were still Good Samaritans.
He said it was the first time he had come to someone else's aid - contrary to the supermarket worker's comment - but it probably would not be the last.
Ms Shallue said it was not the first time she had heard of this happening.
People quite often helped out in the stores and goodwill could make a huge difference to someone's day.
"It doesn't happen every day, but I think Kiwis are pretty generous and if they see someone in strife they'll help them out."
See the stories and comments shared under Mrs Hartell's post for yourself on the Wanganui Chronicle's Facebook page.
-With APNZ