OK, this is in response to the people who have posted on this subject. Someone was kind enough to give me the exact details, but I shall protect my sources except to say they know the insides of MS tools well. The warning here is that this is unsupported and could completely destroy your users profiles...
Someone asked me how to do this and I discovered that you can't have a negative discount percentage, negative quantity or negative price. So this makes it impossible to do... well, no, but it is not quite as easy as we would like. To make it work you need to create a Product and Service csv file...