A third variation of the shopping cart DTO - everything at once!

ShoppingCartDtos3.cs

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

  [JsonProperty("unpaidItems", NullValueHandling = NullValueHandling.Ignore)]
  public Item[] UnpaidItems { get; set; }

  [JsonProperty("paidItems", NullValueHandling = NullValueHandling.Ignore)]
  public Item[] PaidItems { get; set; }

  [JsonProperty("payment", NullValueHandling = NullValueHandling.Ignore)]
  public Money Payment { get; set; }

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