Hello, I have noticed Sergey's comment about custom fields in Emerald 10.
I am working on Emerald 10 right now very close to release. This version supports custom fields you can attach to plan purchase.
Apart from the custom fields content for additional infos, do they have other features?
What about adding "Optional Items" to a plan, which might have an additional item price. This price might affect the total amount. Usually increasing it, but decreasing would also make sense. "Optional Items" and their price could be listed on the "receipt/bill" separately. etc.
If custom fields are not suitable for such advanced features, I could imagine to utilize Cross Plans for a similar usage ;)