You would denominate the deposits in USD (and the goods) and perhaps the plucky soul who is running the mint could put up his own funds 1:1 for all deposits and then hold the BTC himself, taking on the price fluctuation risk.

The goal is to save merchants (and customers) the sometimes nearly 4% credit card processing fee. if you wanted to get very creative - after more analysis than I'm doing off the cuff right now - you could have the merchants pay a much lower processing fee, maybe 1% or 0.5%, and that could be pocketed by the aforementioned mint backer to offset his risk. If the Bitcoin price is favorable for a period, maybe he even refunds that fee back to the merchants or refunds + a bonus.

Agreed that the #1 goal is consistency and ease of use (and discounts) for the customers, and #2 goal is savings for the merchants. It's not simple bringing the this lower-fee, instant/guaranteed settlement network to uninitiated merchants and customers; there are some tradeoffs. But if whoever is attempting to run this operation is sufficiently incentivized to see it through, he could take on a bit of risk himself (and thus all the downside in the tradeoff...) in order to help it succeed with good UX.