A company for whom $130 is significant today is not necessarily a company for whom $130 is significant tomorrow. One thing we want to do is lower the barrier to people trying out their ideas, and for those ideas that turn out well we've gained a large customer. For those that don't go well we (and our user) basically break even.
And as well as being a home for the casual weekend project, we have a number of users processing millions of dollars a year.
I would imagine, though, that companies that are "on their game" enough to become big are probably also the kind of company that will realize that they are now being burned on per-transaction costs rather than fixed overhead, and have the wherewithal to spend the week required to recorde their backend for a different API. Some companies won't get around to it quickly (after all: successful companies are busy), but that still seems like an awkward thing to rely on.
So, on that note, have you thought about setting up fees that scale better for larger companies? If you are doing two million dollars a year, PayPal's fees (for an example; PayPal is even a little more expensive than most processors, but are simply so far reaching it is hard to avoid them) will have dropped to 1.9% from 2.9%, saving $1666/month; at $10 million a year, this is enough for a reasonably high quality full-time employee.
And as well as being a home for the casual weekend project, we have a number of users processing millions of dollars a year.