Refactor Code

Refactor Test Code

Rather than repeating the same test/behaviour for different values, we can make it data driven by giving the data in line:

[Theory]
[InlineData("Apple", 3, true, 2.00)]
[InlineData("Apple", 6, true, 4.00)]
[InlineData("Apple", 7, true, 5.00)]
[InlineData("Banana", 3, false, 3.00)]
[InlineData("Banana", 6, false, 6.00)]

public void Apply_BuyTwoGetOneFreeOffer_WhenApplicable(string itemName, int quantity, bool isEligible, double expectedTotal)
{
    _mockOfferService.Setup(s => s.IsEligibleForBuyTwoGetOne(itemName)).Returns(isEligible);

    _cart.AddItem(new Item(itemName, quantity, 1.00));

    var total = _cart.CalculateTotalCost();

    total.Should().Be(expectedTotal);
}

Repeat the tests to ensure they are passing.