Its not even legal tender so the merchant would have to convert back to fiat anyways. If they need fiat to operate their business

The easiest way to do this is through a service similar to credit or debit cards.   Isn't there some service for this already?

Its a pain to use bitcoins currency.   No different if you work for dollars then instead of a checking account you buy gold.  When you need dollars you sell that gold back to dollars to make purchases