What exactly do you mean by "large customers"? We run several million dollars worth of credit card transactions per year( through cybersource and authorize.net right now ), and we pay at or near the wholesale interchange rate.
Even so, our accounting department still can't fully untangle the mess of foreign transfer and conversion fees, float-related costs, gateway fees, amex vs visa discount rate differentials, pci compliance service fees, and all the other horrible shit that Stripe eats for you, in order to give me a straight answer as to whether or not we currently pay more or less than 0.30 + 2.9%.
This is not a hit on our accountants, cybersource, stripe, or our merchant bank - just making the point that it's not so easy to state categorically that Stripe's fees will turn off somewhat larger volume customers.
Generally you end up paying 3.5-4% assuming you accept foreign credit card transactions, Amex, corporate cards, rewards cards, etc. There's a misconception that it's around 2%, but if you do these types of transactions and look closely at your statements, 3.5-4% is the normal range, including a few chargebacks.
If you are indeed close to interchange, you shouldn't have an issue with many of those things (aside from Amex). Cybersource and Authorize.net are a source of your problems - they do their best to lock you in to terrible pricing and tack on hefty fees all over the place.
Foreign fees really should be quite close to your other fees. You take a hit on the fact that they are "foreign" but they actually fall under a lower interchange category, so it comes out a wash for the most part. There should be no float-related costs, gateway fees are standard for auth.net, most reasonable places won't charge you PCI fees if you fill out the simple questionnaire, etc. With a decently-priced interchange plus merchant account and gateway, you should come out much cheaper.
Disclosure: I'm with FeeFighters (http://feefighters.com). We have a bundled merchant account/gateway (http://samurai.feefighters.com) that is a bit different from Stripe, priced at 2.3% + $0.30, but has a monthly fee of $25 and AmEx is separate (2.9%+$0.15).
Even so, our accounting department still can't fully untangle the mess of foreign transfer and conversion fees, float-related costs, gateway fees, amex vs visa discount rate differentials, pci compliance service fees, and all the other horrible shit that Stripe eats for you, in order to give me a straight answer as to whether or not we currently pay more or less than 0.30 + 2.9%.
This is not a hit on our accountants, cybersource, stripe, or our merchant bank - just making the point that it's not so easy to state categorically that Stripe's fees will turn off somewhat larger volume customers.