@Mikael39
Hello Mikael,
you can't set 'bed linens as a 'Extra Charge' but the best way to do this is to put this charge in your lsiting description and when a guest sends a reservation request, send the guest a 'special offer'! When you hit that field you will be presented with a page that looks like this........
Arrow 1........ use the scroll arrows to set the price you would like to send the guest!
Arrow 2 .......with each push of the up or down arrows the price the guest will pay including the Airbnb service fees will be shown.
Arrow 3.......The price that you will receive will be displayed here. You can play around with this until you have the offer that suits you and when you are ready and wish to proceed hit the 'Send Special Offer' field and the offer will automatically be sent to the guest. You can explain in the message stream with the guest that this offer includes the use of articles that are not covered in the automated Airbnb booking program.
Cheers......Rob