Every fuel distributor specifies exactly what they want from the fuel depot. By and large there is only one fuel depot in most markets so all gasoline comes out of the same pipe.
Sometimes one has a choice as to which blend, such as summer/winter/fall/spring.
The distributor always has the choice as to the detergent package. Can't buy the Shell detergent package unless one is a Shell station, but the fuel depot has Shell's detergents and mixes as the delivery truck is loaded.
Top Tier simply stipulates minimum quantities of EPA approved detergent packages in excess of the EPA required concentration. There is no reason everyone cannot use TT quantities without paying TT tithe, just can't use the TT trademark.
The whole purpose of TT is to convince buyers of Costco (and other off brands) is as good as Shell and Texaco/Chevron.