Okay, so DW said that upon passing through security, she went up to the third floor, where she was required to give her DS-260 confirmation page to the gent behind the USCIS window. This is the one on the far side of the room, toward which the seats are facing (Same one where DCFers submit their I-130). Then, she was directed to go to the second floor to pay. The beneficiary then takes the receipt up to the third floor and shows it to the individual behind the previously mentioned window. This individual asks for additional documents. She didn't remember them all, but said that they included all notarized documents, as well as the I-864 and accompanying tax documents. At this point, she was given a number and told to have a seat until her number was called. Hope this adds a bit of clarity to what it means to "pay before" one's interview.