Merchants are paying 3% or so to accept money from their customers today. There are zero-fee, instant conversions for customers and merchants to use Bitcoin (Circle and BitPay, respectively). Once the user experience is right, customers will pay using funds from their bank account via Bitcoin with near-zero costs (slight exchange rate differences between services), and merchants will offer their customers discounts to pay that way.
Two years from now, merchants won't be paying 3%.