If you missed the original offer, you’re in luck. Amex is extending its offer to promote spending at small businesses by offering a $25 statment credit when you use your American Express card to spend at least $25 at a qualified small business by December 31, 2010. Eligible merchants include any independently owned small businesses that accept the American Express Card. Purchases made online, at large, national chains, at franchised business locations and at government agencies are not eligible. Prepaid and corporate cards and ExpressPay transactions are not eligible.

To get the credit, you must first register for the promotion, which you can do here. Registration is limited to the first 300,000 enrollees. Note that if you’ve already signed up a card for the previous Small Business Saturday promotion, that card won’t be eligible for this extended promo.