I would definitely consider capping the pricing structure. Something like 2% of the first $10k transasctions, and 0.5% thereafter. That or just have an introduc

I would definitely consider capping the pricing structure. Something like 2% of the first $10k transasctions, and 0.5% thereafter. That or just have an introductory tier that's based on % of transactions, and higher level tiers that are monthly fees. The percentage fee is very attractive for small merchants who just want to try something out, but obviously loses its upside if your experiment works out.