Object-Process Methodology (OPM) is a system development and specification approach that combines the major system aspects–function, structure and behavior–within a single graphic and textual model. Having applied OPM in a variety of domains, this chapter specifies an electronic commerce system in a hierarchical manner, at the top of which are the processes of managing a generic product supply chain before and after the product is manufactured. Focusing on the post-product supply chain management, we gradually refine the details of the fundamental, almost “classical” electronic commerce interaction between the retailer and the end-customer, namely payment over the Internet using the customer’s credit card. The specification results in a set of Object-Process Diagrams and a corresponding equivalent set of Object-Process Language sentences. The synergy of combining structure and behavior within a single formal model, expressed both graphically and textually, yields a highly expressive system modeling and specification tool. The comprehensive, unambiguous treatment of this basic electronic commerce process is formal, yet intuitive and clear, suggesting that OPM is a prime candidate for becoming a common standard vehicle for defining, specifying, and analyzing electronic commerce and supply chain management systems.