these come with zero interest

$8 McFlurry likely has the financing baked into the price of the product.

You’ll occasionally see businesses (SPEC’s is the local shop that leaps to mind) that will give you a discount for using a debt card rather than a credit card. That’s because the credit card company tends to charge a 2-4% transaction fee on the purchase. SPEC’s can save money by offering to discount their merchandise by some portion of that transaction fee.

The reverse is also true. A retailer that works with Klarda or some other DeFi platform can simply raise the price of all its products to cover the (typically much higher) transaction cost.

This defers the credit risk (if there’s a 12% surcharge, you don’t mind when 10% of the bills go unpaid) in a system that is highly punitive for debtors and tax-favorable to creditors.