Buying multiple charms from the cottage witch only gives you your first purchase.
I tried a Charisma Charm and a Perception Charm, but only the Charisma appeared afterward.
EDIT: I think I figured it out. I paid the 100 gold for the ...thing you get from her, but my gold count in her shop didn't reflect that. I paid 100 for the quest item, then 200 for each charm, but I only started out with 470 gold.
Most likely it was related to that. I guess this defect is now Buying the McGuffin from the witch doesn't deduct the money before you reach her store page.