As helpful as your step daughter is being, you are perfectly at liberty to do what you wish for yourself, particularly as you are trying to remain within the government's guidelines. It sounds as if she is passing her own 'story' and judgement about the situation onto you. Don't forget you also have your own story and judgements!
Insisting that she does your shopping for you, against your own personal judgement, is actually a certain form of bullying in a strange kind of way, hidden under the guise of 'helping' you while telling you that you should be grateful.
Being older shouldn't mean that you can't think for yourself or do things for yourself. It's your right to feel comfortable with what you choose to do, no matter how helpful or grateful you feel you 'should' be by someone's offer of help. If you are not comfortable with the situation, then it means you are being coerced and that's not fair or right.
Don't forget that as long as you are keeping within guidelines, you have every right to take any path which you feel is right for 'you' without having to feel guilty or ungrateful.
I suggest thanking your step daughter for her valued offer of help while letting her know how grateful you are but while also stating that you would feel more comfortable getting your own groceries during this lock down period.