Not having your groceries or library books spill all over the road is necessary. I've had various systems that did just that, starting with a beautiful set of "saddlebags" my parents bought me in 62. Leather fronts, cardboard backs. I had to hide my books and go back and get them on foot.
The cheapest solution is wald baskets. I have the rear set 535, or a Montgomery Ward copy, 45 years old and still working. The picture of rear wald basket on amazon seems to have left off the bar and clamp that wraps around the down tubes behind the seat on the front. You'll need something up front, if just a seat post clamp. More cutting, drilling and possibly welding. Keeping the clamp off the front brake got to be a pain post 1980 with rim brakes. Sixties frames had more tubing back there. One wald basket needs a 5x17 akro bin in it to control the air pump hose, tubes when the rubber band breaks, the 7/8" wrench for the hub motor nut, my pill bottle.
I also have a set of panniers from yubacycles that cost $100 each and are extremely tough. The yuba's have a bar rivited to the back that I tie to the child carrier rack on my bike with farm store twine. The baskets are easier to load, air transparent when empty. The panniers are lighter and keep the rain off the goods. I had to rig an aluminum strut across the right foot bar on the yuba to keep the pannier from pushing on the shifter, keeping me out of gears 7,8.
Don't ignore the front baskets that come up when you put wald basket into amazon search. Carrying my supplies only on the rear unbalanced my mountain bike to 20 lb front 120 lb rear without me on it. I had five incidents of the MTB or cruiser front tire skidding or whipping sideways on obstructions and pitching over on my chin. The last at 25 mph, finally broke it. Why I bought the extended frame bike left. Puts my weight on the front tire. Equivalent frames come from radrover xtracycle and pedego stretch.
