Stupid question: What if the value of bitcoin changes during a transaction?

I tried to research this but couldn't find any good results since Google is useless now.

I'm new to bitcoin and a website I buy from offers a significant discount for bitcoin. I decided to try it, but because the transactions have to be manually processed, I do worry that the value will drop overnight. It obviously wouldn't be huge but would I have to send him a few extra cents of bitcoin or something to make up for it?