OP, you're being too nice about this, you agreed to the price, now she's trying to get some more money.
Just tell her you are at the maximum price you can pay,and can't afford to pay any more. If she wants to halt the sale to you, let the bag go.
You mentioned that you have slightly overspent anyway, so maybe it would end up being a relief to let this bag go?
I think if we let me monkey business start up on bonanzle, we'll end up in the same pickle we now have on ebay, and none of us wants that!