Walk into, or drive through, your local edition of a national McHorribles fast-food chain, or even a local startup, generic Sack-O'-Food establishment. Of the specifics of your experience, a fair portion I believe I can guess:

About 50% of the time, clerks attempt to put words into your mouth. "Hello, welcome to Sardees, what combo meal would you like to order?" In order to ensure that I do not inadvertently order a MegaBurger Deluxe meal instead of just a MegaBurger Deluxe, the first words out of my mouth are some variant of "No combo!," usually with the addition of some form of the word "damn." (This is somewhat off-topic, but is a related point.)

The drink, a brew mixed together on-site out of proprietary, closed source syrup and carbonated water, is in many cases one of the most costly things on the menu. Indeed, if there is any sort of special on its major component served separate, then the drink is often the most expensive individual element of a combo meal.

After adding up the prices of the individual components of a combo, often times it turns out that the combo discount (compensation for the "exclusivity" agreement) exactly equals the price of one of the elements. The burger, fries and drink combo exactly equals, in expense, the burger and drink alone. When this occurs, the fries can be considered to have "null price". Even if you despise their fries, you're still better off buying them, even if only to pass their value along to nearby ducks or squirrels.

(I will be filled with shame if, in the months following the posting of this writeup, studies indicate a rise in the incidence of heart disease among ducks and squirrels. Nature takes another blow to the gut.)