If you book through Princess there used to be two different tours to get to the falls. One was by bus, which entered at the top of the falls & you walked past all the shops, walked to the bottom of the falls and proceeded to climb. They didn't bother to try & sell you junk on the way in but when you're leaving it was a totally different story. You'll almost be held hostage at each shop on the way out.
If you opt for the tour which goes to the falls by catamaran, it docks at the bottom of the falls & you begin the climb from there. You exit the falls at a point where there aren't any shops & then walk back down to get the catamaran for the return trip without passing them. Of course you'll still have to deal with the guides who will insist on a healthy tip & won't take no for an answer.