@Viktoria56
Alas, you can't just bump up the price of a single night stay by adding a special supplement. There are some possible solutions but a bit nit-picky though.
What you could do is raise your base price to a high enough level that would make it worth your while to host single nights, taking into consideration a cleaning fee.
If your core business is multiple nights, but the price is now too high with the higher base price, you may add percentage discounts for two, three etc night bookings using 'length of stay' discounts. This would give a differential to single nights against multiple night bookings.
You can also consider a second listing; only opening up orphan days in your calendar but always using a higher base price.
