In general, the subject matter described in this disclosure can be embodied in methods, systems, and program products. A server system receives, as sent by a computing device, a request to purchase a product. The request is initiated by user selection of an element provided for display by a third-party application program. The server system sends, for receipt by a transaction processing system, a charge request to charge an account of a user of the computing device for purchase of the product. The server system determines, after the charge request has been sent for receipt by the transaction processing system, to cancel the charge request. The server system sends, for receipt by the transaction processing system, a request to cancel the charge request. The server system sends, for receipt by the computing device, an indication that purchase of the product has been cancelled.