If you have a 6% profit margin (i.e. grocery store) and then you have to pay 3% of basically all revenue, you are now at 3% profit margin. This is why Costco only accepts VISA - they have 11% margins so 3% would be over 25% of their profit. So they go exclusive with one processor and have a very large transaction volume, so they are able to negotiate much lower fees and thus lose less profit.