This can be a policy of our partners. You can find the information in the offer itself. When a room or apartment is occupied by only one person, the property often calculates a surcharge. This is because the accommodation often allocates one person in the same room or apartment as two people, and therefore needs to heat, light and clean the room depending on the room size and not on the number of occupants TravelBird, unfortunately, has no effect on the level of these surcharges.
Airline companies don’t calculate a single person surcharge/supplement, so single travellers will only ever have to pay for one (return) flight.