Wonky.Client/Wonky.Entity/Models/Cart.cs

using Wonky.Entity.DTO;

namespace Wonky.Entity.Models;

public class CartItem
{
    public int Quantity { get; set; }
    public SalesItemDto Item { get; set; }
    public decimal Price { get; set; }

    public decimal Total
    {
        get
        {
            var price = (from x in Item.Rates where x.Quantity == Quantity.ToString() select x.Rate).First();
            if (string.IsNullOrWhiteSpace(price))
                price = Item.Rates[0].Rate;
            Price = Convert.ToDecimal(price);
            return Price * Quantity;
        }
    }
}

public class Cart
{
    public List<CartItem> Items { get; set; } = new List<CartItem>();

    public decimal Total
    {
        get
        {
            return Items.Sum(item => item.Total);
        }
    }
    public DateTime LastAccessed { get; set; }
    public int TimeToLiveInSeconds { get; set; } = 60; // default   
}