NEW YORK ( LowCards.com) -- Twitter is entering the growing mobile e-commerce market by expanding its relationship with American Express ( AXP) in an offering that starts today. Amex Sync will allow American Express cardholders to make purchases on Twitter through tweets that include special hashtags. The first offer will be for discounted AmEx gift cards, but the program is to expand soon to include physical products from Amazon ( AMZN), Sony ( SNE) and Microsoft's ( MSFT) Xbox. E-commerce sales surpassed the $1 trillion mark last year for the first time, according to a report from eMarketer. Sales in 2012 increased 21.1% versus year-ago levels. Sync is another step by AmEx to merge its services into social networks and grow its share of the contactless payments market. American Express already had a relationship with Twitter, as well as FaceBook ( FB), Xbox Live and Foursquare ( FSQR), that allowed registered cardholders to get online discounts. But this new arrangement is the company's first step in enabling cardholders to actually buy products on a social network. To get started, cardmembers have to link their their eligible American Express cards to Twitter by going to sync.americanexpress.com/twitter and adding a billing and shipping address. Starting today, users can tweet #BuyAmexGiftCard25 to buy a $25 Amex gift card for $15. Users will get a confirmation tweet within 15 minutes; the user then tweets back to confirm the purchase. American Express says credit card information is not shared with Twitter or merchants.