@Bronwen3, @Michelle-And-Michael0, @Lynette57 @Christine615 @Sarah977, I think people are missing the point. What you're saying is that when only two people book, you want to charge extra if they use both beds instead of only one bed. In my view, if your accommodation was at a budget rate, I would request an additional charge for use of the extra bed. However, it seems like your price is already quite high (forgive me if I'm wrong, I don't know what the going rate is in Camden) so, based on this, I wouldn't be too concerned if people use both beds. I would just include it as part of the overall charge.
However, if you do want to charge extra for 2 people/2 beds, then you don't have to go through the resolution centre to do so. Clearly state on your listing that there will be an additional 'linen' charge for two people who use both beds and that guests must request the extra bed before booking. Also, double check with guests when they send you a booking request or instant book whether they need one bed or two (so that you can 'make up the bed for them'). If they do want the second bed, send them a Special Offer (bear in mind that this must be done prior to booking - you can't do it once a guest has booked). Do not provide linens for the sofa bed if the guest hasn't paid for it. Slightly cumbersome I realise, but much better than going through the resolution centre as you may or may not get paid and it would appear somewhat deceptive to guests to be charged extra after the fact.