Top Tier fuel retailers use a higher level of detergent additives compared to other retailers who are using the minimum EPA required detergent additives in order to help prevent the buildup of engine deposits.
In my market, Sam's Club gas prices are often 10cts/gallon less than other brands that are listed as "Top Tier". I've never seen Sam's listed as a Top Tier fuel, nor have I seen Sam's represent that the fuel they sale is equivalent to or meets any requirements for "Top Tier".
It is generally understood that most of the gasoline for any market comes from the same pipeline or storage tank. This gasoline is a commodity and meets any minimum requirements. The gasoline is transported to the station via tanker trucks that are usually carrying a bit over 8,000 gallons of fuel. So how does one tanker carry gasoline that is "Top Tire" and the next one not? Or the same driver and tanker could conceivable carry one load that is "Top Tier" and drop it at one station and circle back to the same loading rack and the next load would not be "Top Tier"?
At the loading rack that I was familiar with, there were what I will call much smaller satellite tanks adjacent to the loading area. These additive tanks were piped into the loading rack and contained the fuel additive. Major Brand X might have their own tank and Major Brand Y might have another tank etc. Both the additive equipment and product was owned separately by the respective major and would automatically be added "injected" into their fuel loads.
The invoice would either billed the fuel in gallons times the price and another line item for freight (transportation). The transportation charge varied by distance - usually a fixed cost for any destination inside the metro with the cost going higher for destinations outside of the metro - the farther distance in miles the more the transportation charge. Or for the majors, the fuel quantities were just recorded as an "exchange". No money changed hands by truck load but it was an inventory adjustment item. For example, I picked up so many thousand barrels of product at your terminal X and you picked up so many thousand barrels of product at my terminal Y (across the state or region) and we keep a running record each month of the balance. A terminal may ship by truck, pipeline, barge or all of the above - it made no difference; but the additive would only come into the picture for product trucked to a retail destination since you obviously don't run a barge down to the retail convenience store.
There was never a separate line item on the documents for any "additives" because each company took care of their own. (This is from my memory and years ago so some details may have changed but I suspect it's still pretty accurate)
https://www.consumerreports.org/car-mai ... tra-price/