You can create upsell item to collect any mandatory fee from your guests. There are many advance filters attached with upsell, which you can use to ask for mandatory fee only from specific type of bookings eg. Only Airbnb Guests.
You might notice that you are unable to select the correct currency for your upsells on the dropdown when creating the upsell, and can only select $ or %. The dollar sign is just a symbol to select if you want to collect exact amount for the upsell. Once guests purchased the upsell, the currency used will be the currency of the property. Just set your price and it will take the currency of the property.