We often take it for granted that fruit and veg come in prepackaged amounts, but you can just buy one banana or two carrots — you don't have to buy a bunch of six or a kilo's worth!
This is more straightforward if you shop in person, as you can choose the individual items yourself — and using the scales to print out the cost is actually quite fun.
Persevere if you shop online, though, as you will be able to find loose items with a bit of careful searching — just make sure you select "items", not "kgs"!