We did the take pictures and then wait till the last day thing.
Most of the time she was more happy taking pictures than getting merch.
We did give her an amount of her own money leading in to the trip and made sure she knew how many ‘notes’ would get used to take up the presents and how many she would have left if she bought a present. (all notes were the same amount and we would make up the difference just to keep the visual aid in check)
She might not have understood the amounts but she understood the visual ‘this is all you have left if you get that’.