Following a year of on-again, off-again speculation about its plans in online payments, Google Inc. launched a transaction service Thursday with 92 online merchants of various sizes already signed on, from Ace Hardware, Ritz Camera, and Buy.com to Trendy Togs and FaucetDirect. The new service, called Google Checkout, lets Internet merchants accept card-based payments at a rate that for many undercuts PayPal and other competitors. In addition, in a move some see as calculated to support Google’s online advertising business, Checkout allows sellers that make heavy use of the online search giant’s AdWords service process at least some transactions at no charge. It also extends limited chargeback protection to e-retailers, guaranteeing payment in cases of unauthorized card use or when consumers claim shipments weren’t received.