This feature is under review.
I think for this to work across all bookings the best place to provide these upsells is on the booking confirmation page (ie. post booking).
Any objections to this?
If you have a list of upsells you'd like to provide that'd be very helpful!
And how configurable? I imagine you'd need the option for flat fee per item, fee per guest, fee per guest per night, fee per stay? What else?