If a developer wishes to use the XML Web Service to save an Item (or multiple Items) with a booking, then they must use an Item ID that is already entered into the PMS. While the PMS allows a User to write an ‘extended description’ and save when applying as a Folio 'Charge', the XML Web Service doesn't have that ability to over-ride the ‘extended description’.

However, since most developers would use Inventory Items already set in the PMS (with proper description, cost, etc) this should not be a limitation.  See this article for details on Items that a property has decided are 'Live' and which are set 'Not Live' to sell online.

However, where a developer would prefer to build Packages and Items 'on the fly', we recommend using MyGuest.  With this product, you can place as many 'Guest requests' as needed, and with whatever notes describing the Item(s) desired.  Because each MyGuest Request is saved with the Booking ID, these Requests are linked from the Booking, and in front of staff on the Front Desk, these 'Requests' make ideal places for getting creative with extended info about Item(s) placed onto a Folio.

If you look at the MyGuest Pubic API, it shows a Notes:  element that would be used.  Anything you place here is free form text, thus flexible and availale for staff for tracking and reporting purposes.  In addition, you can use the Request ID to 'consume via the API' tho constantly update the Request with additional details as the Item is sold, altered, deleted, and ultimately delivered to a Guest.  

{
  "hotel_id": 2,
  "room": "32",
  "booking_id": "1332025-4X",
  "guest": {
    "email": "joeb@banks.com",
    "name": "Joe Banks",
    "phone": "7075551414"
  },
  "guest_notify": false,
  "form_category": "Treatment",
  "form_name": "Spa Menu",
  "form_fields": {
    "Notes": "We'd like hot towels and a room to meditate in, which will need to be large as we bring our dogs and cats on meditation retreats with us. ",
    "swedish1": 2,
    "pregnancy2": 1
  }
}