Airbnb sellers can charge a fixed fee directly by setting minimum stay lengths or by charging different nightly rates based on the length of stay (e.g., weekly and monthly stay discounts).
Edit: At most latter is possible with complex rule sets. But I'm really not sure: https://www.airbnb.com/help/article/2061
What I do see is some booking automation that will dynamically set minimum stay lengths to avoid filling, say, a 3 day gap with a 1 or 2 day booking if it’s far enough out and set minimum stays to the duration of the gap.