Small business owners in particular, have to be a bit astute. Simplistically, they will simply note that almost no-one uses Amex with them, or rather, no-one wants to pay the surcharge so they use a different form of payment. On its surface this would appear to work in the businesses favour. You get a cheaper transaction and made a percent or so more margin.
But really, whats going on is that Jaycar simply won't know that the 30-40K a year that we put through Altronics is business they will never see because of the Amex problem. If a business is actually losing money on retail sales by accepting Amex, well, fair enough, don't accept it or surcharge if you must, but I'd hesitate to say that there are more deep seated problems with a business model that can't withstand a 2% hit in order to gain more business.
Now, of course, 40K a year won't keep Altronics doors open, but we can't possibly be the only business in Australia who uses a corporate amex purchasing card. Multiply things out and the income that Jaycar is now signalling that they no longer want is probably a sizeable sum. Not only that, but business (in particular) who use Amex for purchasing are likely exactly the customers you'd actually want. Relatively few purchase orders for a relatively large sum each, rather then nickle and dime orders from hobbyists.
I've used Altronics and Jaycar in my post, but really, it applies to almost any situation and company. Restricting or surcharging common payment forms has got to equal choosing to restrict your customer base. What business, who runs profitably, would want such an outcome?