I don't believe for a second that hundreds of people have bought the advent calendar based on her ad. For several reasons:
1. There's a huge range of advent calendars and the ones at that price point are super competitive. People who are into advent calendars know how to get the best deal in terms of product-to-value ratio (the Cult Beauty calendar is usually the best bet). Other people who don't want to spend that much (which will be most people this year!) will go with something like the M&S advent calendar.
2. She simply doesn't have enough followers to convert her ads into hundreds of sales for something at that price. That's obvious from her general like/comment counts, which are not particularly impressive unless she posts something very dramatic.
3. Lots of much bigger accounts than Jemma's will be promoting the same thing to a much wider audience, and even accounts with 1M+ followers would struggle to get hundreds of sales.
At best, MAYBE a hundred or so people clicked her link to look at the calendar, but that in no way represents the amount sold.