People at volunteering upsetting me

There’s a guy at the charity shop who constantly only talks about using the till to me. The manager told him to stop a few weeks ago (after his actions led to me having a mental breakdown) but he still does it. Literally as soon as I walk in the shop, every week, he starts asking if I have used the till since last week and making me feel pressured. And he laughs at me and says things like "I told you, you wouldn't go on the till". Today I stood up for myself and told him to stop. He apologised and said sorry for upsetting you (he keeps apologising and then bothering me about the till again). How many times does he have to be told?
I am very mentally unstable so working on the till is not my priority. I only volunteer to get me out of the house and to do something ‘positive’!

Then, a lady there who I thought was my “friend”… well I have only known her for 2 months but her birthday is coming up so I gave her a card. Today she was complaining that I didn’t put any money in the card like someone else did!

I am so annoyed. This charity shop is for a mental health charity but some of the ‘staff’ there make me feel so s***.
I am really considering finding a different place to volunteer.

You're volunteering, so you have no obligation to be there. Tell the manager they've made you unhappy and then leave. Make a point and tell them they work for a mental health charity but have, ironically, not helped your mental health. No point staying and letting them walk over you. By walking away you show you can do better than this place or these people. You demonstrate your personal standards for how you wish to be treated