All the information on HW about the ferries from Trelleborg to Germany states that you pay per car not per person. I just had to pay 195 Swedish Crowns (20€) at the port for adding myself to the number of people in my driver's car. (Trelleborg --> Sassnitz) Either, Stena Line fucked me over or they changed their regulations. I actually payed almost double as much as the passenger ticket costs.
For now, I just added my experience as a comment, but if other people make similar experiences we should consider changing the info on this article.