Of course it can drop below that mark, but that is usually a negotiation between a vendor and the payment processor. If you are small and unlikely to drive much business, you will have a hard time getting a lower number. Where I work, we get 2.3% and do several million dollars a year in transactions. We are also a government institution, so it's likely that the processor sees us as a lower risk (though most of the risk comes from the card holder) hence we get the lower rate.

I'd bet that WalMart pays significantly lower fees due to volume.