With the resurgence of the Invoicing module in Sage200, this functionality is available if you have a Financials only licence. The Functionality of Quotation and Pro-forma lives in SOP, therefore part of the Commercials area. People using Financials will still need functionality of recording quotations and proformas if they are not using SOP. I believe it would be beneficial to add this option to invoicing as it still should work without stock and is a bit of a hole in functionality.