I have a chrome extension that I think is fairly useful. It's already been posted and is free. How can I start charging for it with the Chrome webstore? $.99 or some such.
If the extension is already free and you didn't note that there is a "free period" when it was originally posted then you might be better off updating it to be donation-ware and providing some sort of link to a "tip jar" that people can donate a buck or two towards. Likewise, if there is an improvement you could make to the extension that would be a major difference, you could release it separately as a "pro" version and charge for that one, but only if there is a significant usefulness to the newer version and you are still supporting the free version.
I don't think that would work out very well. There are just too many extensions that look like they'll be awesome, but later prove to be not so awesome. I wouldn't pay for something I couldn't try for a while first.
If an extension does make my job easier, I'm happy to donate far more than your suggested price of $0.99, I usually send someone $5 - $10 and a nice e-mail. I've sent people cash for useful greasemonkey scripts in the past.
Just let people know that you're much more enthused about making improvements if the extension buys you a nice lunch a few days a week.
It's yours!! If you feel like charging for it, then Charge for it!
What's the worst that could happen? The people who weren't paying for it will decide to not buy it?
Don't do it bro! Chrome webstore is pretty much free right now. Plus...who is really going to buy something from the chrome webstore...