Hi,
I need to have some extra ability to add negative price in order edit page!
We have a Drink shop and deliver drinks in our area! so our customers order some drinks! for example Cola, on delivery most of our customers have empty bottles and in Germany most of those bottles have deposit (Pfand in german), so we should return them some amounts! and because of that we cannot using coupon systems in magento, and the bad news is we must put those information's in our invoices 
So how can I do that ? is there any module or way to give negative value to a certain product or add discount amount manually directly in order edit page ?
Thanks a lot 