Costco can determine what their target softness is. Many of the technology patents that applied to making soft tissue have run out and now there are a lot of paper machines out there that are capable of making a soft sheet of paper. That being said, most of the big players have strong R&D groups who are very secretive about any new process improvements that will cost effectively improve softness, or give them an edge in the market.
But generally, Costco can say that “We want a sheet of paper as soft as Charmin” and they will find a manufacturer who will make that for them.