The dollar price is there for a guide and should be disregarded when paying a vendor, just concentrate on the price in bitcoin and send the bitcoin amount, if you have underpaid it will be a very small amount, to find out how much subtract the amount you have already paid from the bitcion price of the product then contact vendor and ask how to make up the small difference, they may say something like leave 1 or 2 tips which would cover it, good luck :)

Payment is is bitcoin so you have to send whatever the bitcoin amount is (remembering that mining fees are on top of this). Sometimes the price can move during the transaction. Ask the seller the best way to resolve although this generally requires you to send the balance....