An electronic commerce system includes a client computer and a server computer interconnected by a public packet switched communications network. The client computer is programmed to transmit to the server computer an order acceptance request that includes a plurality of terms or conditions of a proposed offer for a purchase, including multiple options of at least one of the terms or conditions of the offer. The server computer is programmed to process the order acceptance request based on pre-programmed criteria and, based on the processing of the order acceptance request, to transmit to the client computer an order acceptance response that includes a plurality of amendments to the proposed offer for the purchase, including selection of an option of the at least one of the terms or conditions. The processing of the order acceptance request is performed by a controller module that handles processing of the order acceptance request that is primarily not specific to a particular application of the electronic commerce system to which the order acceptance request pertains, and that initiates a plurality of calls to a plurality of plug-in modules. The server can handle fraud-avoidance processing of the order acceptance request. The server can initiate a call to a database of a virtual warehouse in which merchants store virtual inventories of items, to ensure that a sufficient virtual inventory exists for a purchase.