nice idea :)
Kawai Crypto@fluffywiggle does that using Metal and a few other crypto's she supports on her
People can pay in fiat, Steem, etc., but get reduced prices going anywhere from 10 to 30% if they use certain other cryptocurrencies to pay with.
Sadly she doesn't use Steem as one of those currencies X/
This shop would definitely be focused more on Steem and SBD, and trying to encourage using them for payment.
I think the reason why there has been such a big discussion about getting the SBD pegged back to the dollar is that it would allow it to more easily be adopted for payments in commmerce. At the moment the big fluctuations in price would make it a bit risky to use.