Last year American Express trialled a Twitter-based social commerce service, enabling Amex cardholders to get discounts on purchases made with their card (synced to their Twitter account) if they tweeted about them (using a hashtag).  The discount was applied directly to their Amex statement.

Today, Amex has upgraded this save-by-tweet into a pay-by-tweet service, emulating a similar service offered by Chirpify.

Here’s how it works, Amex cardholders sync their Amex card with Twitter at sync.americanexpress.com/twitter. Then, when Amex/Amex retailers offer deals (published in the @AmericanExpress Twitter feed), cardholders can buy them by simply tweeting the deal’s special hashtag – e.g. #BuyAmexGiftCard25.  This effectively promotes the deal – and Amex cards – to all the cardholders followers. To avoid errors, Amex tweets back to the user (@username) to check they want to buy, and the user must confirm with a tweet reply within 15 minutes. The cardholder’s card is debited, and the purchase sent out.

The upgraded Amex pay-by-tweet service is launching next Monday in the US and going live with a deal for Amazon Kindle Fire HD for $150 and an Xbox 360 4GB console with a three-month Xbox Live subscription for $180.

So will it work?  The Amex pay-by-tweet initiative is part of a broad industry move for financial services companies to get more intimately involved with e-commerce. For example, the world’s second largest lender.  China Construction Bank is reinventing itself as a fully-fledged e-commerce portal that is home to thousands of vendors, big and small.  And Twitter powered commerce has precedent, Innovative Thunder’s free-for-a-tweet (paywithatweet.com) service has been popular – winning a marketing award at Cannes, whilst PayPal-connected Chirpify has long allowed consumers to buy and sell directly in-stream Twitter (and Instagram).

But linking payment cards to social media accounts has had a rocky history.  Remember Blippy, and the original Swipely?  That didn’t end to well; surprisingly (not) people didn’t want to share their payment card purchases with the world.

Perhaps times have changed, but othe Amex Sync service on Twitter will only succeed if the deals are really great – worth talking about, and there is a cast iron guarantee that if an account is hacked – or “borrowed” by your sister – the user will not be responsible for unauthorised purchases.

What do you think – would you buy with a tweet?