A transaction processing service operates as an intermediary between acquirers of financial transaction requests and issuing institutions that process the financial transaction requests. The intermediary receives an authorization request generated based on a transaction initiated by a customer at a point of purchase. The intermediary provides a rules module that stores a set of rules for processing transactions. Each rule specifies one or more conditions to be tested and one or more actions to be executed based on the test. For each authorization request, the system determines the applicable rules and tests conditions for each rule prior to the transaction. Based on the results of the test, the system executes an associated action. Conditions may be specified based on transaction information, customer information, or other information. Actions define the system's response to a particular result in testing a condition. Possible actions include rejecting the authorization request or specifying a verification procedure to execute.