Applying Discounts & Coupons - Tracking ROI from promotions

We have a requirement to be able to apply discounts/coupons. Would be great to be able to apply a discount at the line item level and total invoice value when raising the invoice.
We also use the wordpress API's to integrate web bookings into our courses. There currently is no way to run a promotion and offer a discounted rate for a target group as the API's do not allow discounts.
A discount report would also be handy to track ROI for promo's.