Welcome to the world of saavy SaaS businesses. Smart/enterprising SaaS founders and businesses these days all know to tie their billing to whatever metric/KPI they are solving for customers -- per-seat billing, per-pageview, etc.
Why make a little money (even if it's 2-10x your cost) when you can make much more by taking a small-seeming percentage of every single that comes through your business. When your customer wins (and when they aren't winning you aren't a huge strain on their wallets) you win, and if the 0-5% eaten up by their bundle of SaaS products never matters to them, then you get wildly rich doing it and no one is angry about it, since the incentives are aligned. Then you plow that extra revenue (that you wouldn't have gotten from the simple 2x-10x your cost) into expanding your service offering so you deliver so much value (or perceived value) that moving away from your service just doesn't make sense.
This is HN and just about everyone and their mother knows this at this point, and has been living it for longer than I've been writing computers but I'll say it anyway -- Stripe is the dream SaaS company, essentially a financial middleman that no one hates (yet?). The SaaS promised land is a somewhat close cousin of the dystopian everyone-is-a-renter-and-only-a-few-own-the-capital future. Maybe the hand wringing I'm doing is wrong -- maybe it should have been the case all along that solution providers should have extracted a never ending royalty on the revenue generated by the solutions they provide. Not only service providers but maybe workers should want that (no one remembers pensions, but they kind of were this) too. Who knows.
If you can sell a music single album to 0.3% of America for $1, you get $1MM. If you do that with a Stripe store, Stripe gets ~$30k, for infrastructure that probably cost them less than $300 to host (for a single customer) and an ever-decreasing amortized amount to build (assuming they eventually stop growing their development workforce for growth stake). The artist that just made $1MM probably doesn't care about $30k as a millionaire (maybe they should, maybe they shouldn't).
It's a win-win, and it's why Stripe's goal is to increase the GDP of the internet.