How to test bussiness logic in method?

I have such method in my service layer:

public Long calculateItemsCostInShoppingCart(Long shoppingCartId) {
    List<Item> items = shoppingCartRepository.findAllItems(shoppingCartId);
    Long cost = 0L;
    for (Item item : items) {
        cost += item.getPrice();
    }
    return cost;
}

And I need to test calculation of summary cost of all items in list. I was thought about mockito, but it didn't work out cause mockito just create stubs, I need real entrance data and result based on them. How can do it?