If you use their cup for ice, even if you use it just to fill your cooler and then throw the cup away, you gotta pay for that cup of ice. Not only is there a cost associated with the cup, missing cups (from the register and thus from inventory) have a dramatic effect on the bookkeeping numbers regarding food costs and fountain drink syrup yields. It's the result of a quirky way that restaurants and convenient stores track profit and loss, as well as syrup and cup inventory.
Surprisingly, because of the quirk, and it's something they won't tell you, using your own cup for ice and filling it with a fountain drink, and then not paying for it, has a negligible, inconsequential effect on the above numbers. The actual syrup cost of an average fountain drink is between 7-11 cents, depending on the type of syrup. It's not high on their list.
I have one of those stainless steel, double walled tumblers. Mine is 64 ounces. It is a
Rhino Zing brand, with the straw lid. When I fill it with crushed ice, I can add almost 20 ounces of liquid. Cubed ice I can put more liquid in there. But it keeps ice for 24-36 hours depending on ambient temperature.
The wire rack sitting there under the drink nozzles is something I usually have to remove in order to get my cup under there to get ice and to fill it with Diet Dew. After doing that, I will usually (not 100% of the time, but at least 99% of the time) also buy a couple of liter bottles of Diet Dew, because I don't quite feel right about just refilling my cup with ice and Dew and walking out, even though they don't really care. I did the same thing back when I used one of those big 64 ounce mugs you can get at the truck stops. I set the 2 bottles of Dew on the counter next to my 64 ounce cup, sometimes with my cup a little off to the side, they ring up the two bottles and virtually never ask me what I have in the cup. Once in a while they'll ask me what's in there, or ask "Hot or cold?" and I'll answer "Ice," which is technically true, since there is ice in it. They never charge me for the drink refill. I haven't paid for a drink refill in years.
I realize that I should feel bad about this, but knowing that I'm paying twice the cost for the liter bottles of Diet Dew that I would at Walmart or Dollar General, and knowing that I'm putting in about 20 ounces at a cost of about 5 cents, I don't.