The btc got for the merchant's invoice is sold as soon as the btc transaction is confirmed. Just the merchant getting the INR will take upto 3 days. Merchant will get exactly the INR he has invoiced for his shoppper.
If you are buying a computer for 25,000 INR and if you pay using bitcoin, then the merchant get exactly 25,000 INR with in 3 business days (it will usually be next business day though).
So absolutely no risk of volatility for merchants.