Commercial-tour tickets often are available for some time after the (usually much cheaper) tickets are sold out on the official website.
Note, though, that in this case--as explained above--you may not be getting much for that extra expenditure, since regular entry tickets include a guided tour of the camp. I would hope a commercial tour would have a guide on the bus to and from Auschwitz, imparting background information. On the other hand, you'll be stuck with the tour's timing rather than being able to choose your own time for return to Krakow, within the limits of the public-bus schedule.
Viator doesn't conduct any tours. It is just a third-party middle man selling tickets to tours conducted by others. You might save money, and the tour company will likely make more money, if you book directly. The name of the company actually operating the tour can be found in the Viator listing under "Additional Info".