A checkout system for handling sales or payment transactions with a computer module for electronically processing payment-related data, with at least one peripheral module which is connected electrically and mechanically to the computer module, wherein a housing wall of at least one peripheral device adjoins a housing wall of the computer module such that the mutually facing housing walls of said computer module and said peripheral module are arranged parallel, characterized in that the peripheral module (3) is in electrical contact with the computer module (2) and/or with a further peripheral module (4) by means of an electrical plug device (23, 28), wherein the electrical plug device (23, 28) has at least corresponding plug/connection elements (24, 29) which are respectively arranged on the mutually facing housing walls (14, 15) of the adjacent modules (2, 3, 4), and wherein the peripheral module (3) can be mechanically locked to the computer module (2) and/or to at least the further peripheral module (4) by means of a latching device (18, 25).