Order

Order.cs

public class Order
{
    public TimeSpan _purchaseSpanLimit = new TimeSpan(2, 0, 0);

    public Guid Id { get; }
    public List<Item> Items { get; }
    
    public Order()
    {
        Id = Guid.NewGuid();
        Items = new List<Item>();
    }

    public void AddItem(Item item)
    {
        var firstItemPurchasedAt = Items.OrderBy(i => i.PurchaseTime).FirstOrDefault()?.PurchaseTime;
        var totalPurchaseSpan = item.PurchaseTime - firstItemPurchasedAt;

        if (firstItemPurchasedAt == null || totalPurchaseSpan <= _purchaseSpanLimit)
            Items.Add(item);
    }
}