ImageThis week American Express and Twitter announced a partnership to allow people to buy things by tweeting.  It works like this: You sync your Amex card to your Twitter account, and then you can start making purchases by putting hashtags in your tweets that correspond with special deals American Express is offering.

You send your tweet out to your followers—“So excited to wear my Donna Karan Urban Zen bracelet while playing Halo 4!” or something like that—and include the hashtags.  Amex sends you a confirmation tweet, and when you respond to it, the transaction is complete.

American Express is betting that there’s a whole market of consumers who treat shopping as a sort of performance, who like spreading the word about good deals and great finds.   American Express is confident that the rest of us find those people’s advice and tips valuable rather than a nuisance.

According to Leslie Berland, who runs digital partnerships for American Express, that confidence comes from previous collaborations with Facebook, Foursquare, and Twitter itself. (It was possible even before this week to get Amex deals by tweeting hashtags, but you had to go to the store or store website to redeem them.) “Customers loved sharing with other people,” Berland says. “There was so much viral activity, we saw new customers who walked into establishments, and those who redeemed the offers went back again.”

By wrapping the entire purchasing process in a hashtag, the service makes buying fast and easy. If it takes off, we could see hashtags for products in commercials and print ads.