Carradice Super C and Ortleib roller tops both work well for shopping.
You need one single compartment, a heel cutout profile, a locking, quick-release hook such as Rixen and Kaul.
You may want to overload your bags with special offers or bulky items. Flaptop and rolltop closures can be overloaded. Zippers cannot.
You need a rack that is compatible with the mount. In theory the hooks are made for any standard rack BUT there are lots of modern racks which use stupid design features so the hooks dont work.
Overloaded luggage can obscure the rear lamp. The best location is a light bracket at the rear of the rack.
If you have mudguards, then an open racktop is better than a solid top-plate, it is more versatile for lashing bog-roll and other big items.
Alternatively, you could use a trailer.