It really is a great site! No you dont pay anything. Basically they buy your clothing design off you if they think it is popular enough for them to sell. So if I get that accepted, I get $2000 and some vouchers for the store. But I sell the rights to that design as clothing. I still own the rights to my art, but they own the rights to put it on clothing.
But to get accepted, you have to have people vote you highly so that they think the design is popular enough that they can sell a shed load of them, if not they aint gonna take it on.
Its an awesome site and they do stock some awesome clothing!