It's retail.
I don't understand why people object to haggling - or negotiating a price - just because it's for charity. If it doesn't sell and the shop can't turn over new stock, it isn't helping anyone.
Though they are nominally a chain, they are individual shops, which isn't the same as being Debenhams.
Therefore the manager should be able to negotiate in order to secure the best price for an item and to shift it to secure space for new stock. If she can't, management is tying her hands. If she won't, then she's not a good small retailer, which is what charity shops are.
The most important thing is a manager who can price things at the optimum level. That is a rare skill.
If she thinks she can sell it at a higher price than you're offering, then she should. If not, she should come down a bit.
No one gets any prizes in retail for doggedly sticking to the price and getting lumbered with a load of old, tired stock and no room for new stuff so customers get bored and stop coming in.