Final answer:
The suitable MIS tool for a CRM system that needs to handle and analyze large amounts of customer purchasing data is the Decision Support System.
Step-by-step explanation:
An online mail order company decides to create its own Customer Relationship Management (CRM) system which is crucial for analyzing customer purchasing patterns. The company requires a Management Information System (MIS) tool that complements its CRM system by enabling access to and analysis of significant amounts of customer data. The MIS tool suitable for this purpose is a Decision Support System (DSS), which includes human-software interactions and aids decision-making processes, thus supporting the CRM's need to manage and interpret large data sets for strategic and operational decision-making.