[krrrh]

  One caveat: at this time, Bitpay functions in the same way 
  as the other credit card processors on Shopify,
  meaning that you can't use Bitpay in conjunction with 
  Authorize.net, Stripe, or other credit card processors.

This is so half-assed, and frankly sort of typical in terms of the strange limitations that Shopify saddles you with (similar to how it took them 5 years to fix their database so you could modify a shipping address as a merchant). Sure, it would be fun to have a button that allows a customer to pay with bitcoin if they want to or are able to, but how many merchants would forgo credit card processing in favor of bitcoin? This "feature" is link-bait, and nothing more at this time.