this tried and true top notch Tubus rear rack can be had for about 100 U.S bucks ish.
https://www.amazon.com/Tubus-Cargo-C...4L8?th=1&psc=1
an aside--buying groceries has often ended up being with my rear panniers being waaaaaaay waaaaaaay heavier than my rear panniers on a touring trip.
Groceries can get pretty darn heavy pretty quickly, milk, fruit, veggies, cans of stuff.....
also, when touring, we tend to have front panniers also that spreads the weight out, and also are pretty careful of not bringing the kitchen sink.