I say that it depends on the "unit" you're trying to test. Is it large and complex enough in itself that you need to isolate it from other components (dependenc

I say that it depends on the "unit" you're trying to test. Is it large and complex enough in itself that you need to isolate it from other components (dependencies)? For your use case, if the shopping cart is simple enough, I'll just choose a simple or frequently used discount calculator and make some unit tests to test the shopping cart functionalities. Then for the rest of discount calculator cases there will be separated unit tests. The problem with testing almost everything in isolation makes refactoring some small-inner components costly, since every change may break some of unit tests.