Most dessert counters end up selling coffee eventually. It starts as a favor to the one customer who asks, and a year later there is a machine and a queue. At that point the cup stops being an afterthought.
The difference is a sleeve
A single wall cup is one layer of paper. Hot liquid on one side means heat on the other, which is why single wall service usually arrives with a sleeve slid over it.
Double wall kraft hot cups build the insulation into the cup itself, so no separate sleeve is needed for coffee and tea service. One item instead of two.
Count the sleeve before you compare prices
This is where the comparison usually goes wrong. Shops line up the two case prices, see the double wall is not cheaper, and stop there.
At 12 oz in kraft, the single wall and double wall cases are the same price. Which means that once you add a sleeve to the single wall option, the sleeved cup is the more expensive of the two, not the cheaper one. The comparison only favors single wall if you were never going to sleeve it at all.
When single wall is still the right answer
Cold drinks. If the cup is going to hold iced coffee or a soda, insulation solves a problem that does not exist, and you are paying for a second layer of paper to do nothing.
Branding is the other case. A plain white single wall cup takes a sticker, a stamp or a printed sleeve, and for a shop that changes its look seasonally that flexibility is worth something. If you would rather have the design printed into the cup itself, that route is covered in how custom printed cups work, though it comes with lead times a sticker does not.
Kraft or white
Both walls come in both finishes, so this is a separate decision rather than a consequence of the first one.
Kraft is unbleached and reads as the eco-friendly choice, which matters if the rest of your counter is already running kraft cups and compostable containers. White is a blank surface, which matters if you are applying your own artwork. Black exists too, at the smaller sizes, for counters running a deliberately dark presentation.
Sizing, briefly
Hot drinks run 12, 16 and 20 oz across the range. Twelve is the standard cup, twenty is the large, and sixteen is the one most shops discover they did not need once they have the other two. The logic is the same one that governs cup sizing on the dessert side, which we set out in choosing an 8 oz cup: two sizes far enough apart beat five that blur together.
The hot and cold cup options sit alongside the dessert sizes in the cup range. If you are adding coffee to a counter that has only ever served frozen desserts, start with one size and one wall type, and add the second only when a customer asks for it twice.