The witch did something perverted to the fabric of space-time and stepped out onto a rural mountainside in Slovenia fifty years ago. Once she'd entered the cave all the trappings of witchood appeared, and she and her forebears had been witching out of this cave for centuries. Of course she arrived in time for the teenage girl wanting a love charm. What's the point of traveling in time if you're going to be late anyway?
The young girl was extraordinarily beautiful, but weighed down by a heavy heart. There was no joy in her, not like the other girls. This one was a pragmatic, practical girl not prone to flights of fancy. In another generation the likes of her wouldn't believe in witchcraft at all. She's the last of an era. The witch was lost in these thoughts and only let out a half-hearted cackled for her, but the girl was distracted as well.
"What can I do for you, my dearie?"
"It's my future, ma'am. I worry about it in this isolated village. Look at me. Look at my beauty. I am so far superior to the other girls around here that I'm practically a different species and I fear I'll never find a husband worthy of me."
"But my girl! There is so much more to life than physical looks! Are you sure you are not focusing on simple superficial things at the expense of things that could bring you more joy?"
"No, madam. I am sorry. I am here to purchase a charmed future. I heard tell you accept oddly shaped rocks as payment, and I've spent all summer collecting them. Here they are." She dumped a satchel of pretty lake stones by the hearth. The witch took her time pretending to sort through them, muttering things along the lines of 'pretty!" or, 'this will do'.
"Well, my girl. Never let it be said I don't give value for money. You get what you pay for with me. Last chance--would you like to change your wish? No? OK. Abraca-pooftie! There you go, hon. You will marry a fantastically wealthy American businessman and will stay married while he ascends to the presidency. Your beauty will carry you all the way to being the First Lady of the United States of America. Will that do?"
"Oh yes, ma'am. That will do amazingly. Thank you ever so much."
The witch vanished all the stupid stones before departing. It's odd, she mused. You can give humans wonderful magic but they'll never believe it if you don't make them pay for it. They get very suspicious of gifts.