That’s just going to heavily incentivize your most successful users to bail. I run a five figure paid email service and I’m using a membership management service that charges flat fee plus 2%, and it annoys me every month. I’ll be replacing them this year. Membership billing plus a synced email list on Mailchimp is really commodity at this point. Doesn’t make any sense to pay a percentage of revenue.
If it was 10%, I’d replace them this week.
I may get pushback because there’s a contingent of people on here who basically seem to think that ever worrying about price for software and services is the wrong move. I think that’s pretty stupid when you’re a small company. It’s stupidly easy for a lot of small companies who are paying hundreds or thousands per month in recurring subscriptions to shave 20-30% off. If you’re a solo founder, that’s a huge increase in your personal bottom line.
EDIT: I'm probably coming across as too negative here, so let me add a few thoughts:
1. You built something useful, launched it, and have paying customers. That's HUGE and puts you ahead of 99% of "startups" so congrats :)
2. I strongly believe in the power of paid publishing and email marketing (it drives almost 100% of my income these days), and I think more cool tools and platforms in this space is excellent. This is the kind of thing that I would use.
My only real issue is just that I think what you're doing with the pricing model is shooting yourself in the foot (since it incentivizes people to switch away) AND it's unfair to users (since switching away is REALLY hard, and you probably know that and are counting on it to some extent).
Maybe charge flat + fee up to a cap? It'd be really comforting to know that I'd never pay more than $495 / month for this or something, and that would only be once I hit $10k / month in revenue or whatever.