Bitcoin price goes up and down all the time. If someone buys something costing 10$, the next day it may cost less or more. So it's a risk for the seller.
why not tie the price to usd or to any fiat because they are stable .
in that way the price of the product will still be the same no matter what happens on the price of btc . the only change that will be made here is the fee , the fee can be expensive or lessen depending on the price of btc .
this is the risk for the buyer and not for the seller anyway . many shops right now are accepting btc . not just btc but altcoins too .