There's a credit risk attached to credit card processing (charge backs etc) so merchant services providers with the lower rates can be picky about who they'll take on. This is why some of those who serve the micro/small business side of the market have lengthy retention periods for release of funds. So a lower rate might come with a temporary hold on your money whereas a really low rate might be unavailable as the retail enterprise doesn't satisfy the required credit profile. Paypal serves pretty much everyone without holding onto funds and, to some extent, if can do this by pricing the risk into it's fee structure.