A second variation of the shopping cart DTOs using custom attributes.

ShoppingCartDtos2.cs

abstract class ShoppingCart 
{
  public ShoppingCart(string state) 
  {
    State = state;
  }
  
  [JsonProperty("_state")]
  public string State { get; }
}

class EmptyCart : ShoppingCart
{
  public EmptyCart() : base("empty") {}
}

class ActiveCart : ShoppingCart
{
  public ActiveCart() : base("paid") { }

  [JsonProperty("unpaidItems")]
  public Item[] UnpaidItems { get; set; }
}

class PaidCart : ShoppingCart
{
  public PaidCart() : base("paid") {}
    
  [JsonProperty("paidItems")]
  public Item[] PaidItems { get; set; }

  [JsonProperty("payment")]
  public Money Payment { get; set; }

  [JsonProperty("timestamp")]
  public string Timestamp { get; set; }
}

class Item
{
  [JsonProperty("id")]
  public string Id { get; set; }

  [JsonProperty("title")]
  public string Title { get; set; }

  [JsonProperty("description", NullValueHandling = NullValueHandling.Ignore)]
  public string Description { get; set; }
}

class Money 
{
  [JsonProperty("amount")]
  public double Amount { get; set; }

  [JsonProperty("currency")]
  public string Currency { get; set; }
}