I'm not sure OP.
Perhaps she could have done a bit more for you, offering to hold the skirt for a certain time maybe. But only if the shop have a policy allowing that.
As for dropping the money in later, I don't think she was being unreasonable to refuse.
We get that a lot at work when people order drinks from the bar. They order and watch you get the drinks, then ask if they can owe you 2p or 5p or 20p or whatever until next time as they don't have the right change.
It started happening so often we had to start refusing to let people do it.
Either they forgot they owed the money, and argued about it when they were asked for it.
Or they remember but get very huffy about being asked for the money in a "can you believe they are chasing me for 2p" kind of way.
Or they claimed they had already given it to one of the other bar staff.
Or everybody forgot until it was too late and the till was wrong.
Or there was a constant stream of "I'll bring it next time…oh, I forget, I'll get it later…sorry, no change this time, I'll pay you later…"
When customers do this to me at work I never offer to put in the difference. I'd be doing it all the time and it would cost me a fortune.
As a customer in the shop with you, I'd have given you 2p at the time.
But as that woman working in that shop, who doesn't know you or your circumstances or intentions, I'd probably think you were one of many people who had said the same thing to her that week, that some people ask for things to be put to one side and then never come back for them, that some people try it on in charity shops to get something for less than it's priced at, that her shop doesn't allow her to do what you were asking as you might not come back with the money, that if you make one exception because it's only 2p you then have to make another exception that might be for £2.00 or £20.00 because you can't pick and chose which customers to favour.
When it comes right down to it, she doesn't know you and she doesn't own the shop.
If she let you take goods without paying in full, by thinking "it's only 2p and she's said she will come back" then she can't really refuse to let someone else take goods while owing any other amount of money either.
She doesn't get to pick and choose which customers are trustworthy enough to return or how much money it's okay for the charity to potentially lose if a customer has taken the goods but doesn't return with the money to pay for them.
So although I can see it was upsetting for your daughter and embarrassing for you, I can see her side as well. She can only do what the charity she is working for allow her to do, and although your intentions were good, she doesn't know you and can't make that judgement, not for any amount of money. She could possibly have offered to hold onto the skirt for a short amount of time, if the shop policy allows it, but equally you could have asked her if that was possible.