There are many billing solutions geared toward SaaS providers, but not many for service businesses that bill monthly (at least in the API sense).
Our product is performance-based, rather than unit-based, and there's no exposed API. We would like to have a detailed itemized invoice (think a phone bill). Our options are build from scratch or shoe-horn existing solutions. It'd be nice to not have to reinvent the wheel and also keep our customers within our UI.
It has a concept of percentage-based billing where you can specify an input amount and you can specify to bill a percentage of that and handles generating a detailed invoice.
Whenever there's no pricing it immediately says "not for me, move on". Maybe that's your intention!
1. Customer original bill (variable - user provided) e.g. 100
2. Saving (variable - user provided) e.g. 9
3. Saving percentage (calculated) e.g. 9%
4. Billed Percentage (variable based on savings percentage?) e.g. 25% x 9
So you would have a billed percentage "rate card" that depends on variable factors like the customer bill plan, the amount/percentage saved etc? Is that the tricky bit?