Allow for quantity modification from the Summary screen
Example: furniture store selling pieces individually, but some are sold as individual parts of matching sets. There is no "minimum" or "maximum" quantity for each individual unit.
Customer #1 is interested in "Alfa" furniture set, which includes Sofa, Ottoman & Loveseat. He should be able to order 5 Sofas, 2 Ottomans and 0 Loveseats without ever leaving the Summary screen.
As a seller, I'd like the option to choose whether my customer clicks into each individual product to modify its quantity, or whether that can be done from summary screen.
If the customer wants to change the color or material of the Alfa furniture set, they can click into each component. But just to increase or decrease the number of each component, it helps sales if it is intuitive and doesn't require clicking through multiple places.