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.
You mention the main risk about accepting coins, if the coin drops 30% then all your prices will drop that percent. And is a not cool loss at all.
For a seller it is not going to have a price drop on their product, obviously their prices will based on their local currency for example USD it will be a fixed value. With bitcoin payment added to their option, it is a risk to their fund, basically it is their total revenue at risk since the volatility is in the topic.
Some people think it's possible if the value of the things is based in dollars and paid in bitcoins, but the only way to make this with low risk is by selling the btc at the moment you get the payment... But even if we do that and the price goes up after we sell then it will represent a loss.
If you are thinking that accepting bitcoin as a payment now, then you're not thinking right. With so much volatility in bitcoin market right now you could be in a deep mud if you are willing to accept bitcoin as a payment for your store. And I don't think people who has bitcoin is willing to spend it because the blockchain is congested and spending/sending it would not be that worthy at all. We can always say that bitcoin isn't going below $2000 or $3000 but man it is bitcoin, if you see it skyrocketting in $20000 then expect it to not plummeting to $2000.