As has been said.. a cheap rear rack and a box/bag that fixes on top of the rack.
This will not effect aerodynamics much and will take the strain off your lumbar vertebrae and 'sit bones'.
You will need to train up to pull the extra weight, or slow down up the hills to reduce the power requirement.
( Everything in cycling eventually comes back to 'Power' as the lowest common denominator

).
I have a BSA 20 1970s shopping bike with a back box to carry my purchases. The bike weighs 34lb unloaded.
I ride it from home to Fred Williams in Wolverhampton, then home via
Decathlon at Merry Hill. The whole trip is 45 miles with some short, sharp hills between Merry Hill and Brum.
Its a four hour trip. 07:00 start Saturday mornings to get to FWs as they open. I just have to take my time and enjoy the scenery ( Cradley heath and Rowley Regis ).
I wouldn't do that trip on a 22lb bike with a 18lb backpack.
