There are so many poorly designed racks and panniers, make sure you get a sound luggage system.
In addition to 2x panniers, I always carry about 2-4m of chord for lashing large items. Chord such as parachord or thicker is better than elastic for big objects. Pick a few useful knots. I always put a loop at one end for a slipknot.
Multipacks of toilet tissue, xl size washing powder etc are too big for panniers so go on the rack top. You can split up multipacks of drinks/cans for easier packing. Take care to avoid hard edges against pannier fabric, use softer shopping or a spare jacket to protect your bags. Obviously, heavy stiff goes first, long stuff (spaghetti) goes vertically along the corner and lighter, more squishy shopping goes on top and your repair kit should not be at the bottom. An extendable collar allows you to overload a pannier beyond the normal closed capacity.