As someone else looking for the same thing, I'd pay a monthly subscription on a per number of tiers. I'd likely pay around $10-15 for every 1,000 users, probabl

As someone else looking for the same thing, I'd pay a monthly subscription on a per number of tiers. I'd likely pay around $10-15 for every 1,000 users, probably a little more. I wouldn't ever do a "% of revenue" or a fee on top of existing transactions.