Have you tried Lago (open source billing)? This is precisely what they do, their HN launch is here: https://news.ycombinator.com/item?id=34773442
They listed the billing engineering nightmares too: https://news.ycombinator.com/item?id=31424450
It's a massive shame that simple table stakes features are behind "get a quote". Folks, I'm not even sure this makes good business sense; just charge me 0.5% of revenue for premium and set enterprise up as 'get a quote' with enterprise features or 1M revenue plus.