1. A computer-implemented method for storing information about transactions between users, the method comprising: as implemented by one or more computing systems, providing a transaction information storage service for storing information about transactions between a plurality of users, the transaction information storage service having multiple computing devices available for storing multiple logical storage partitions for the transaction information storage service that are each specific to one of the plurality of users and include information about transactions involving that one user, the providing of the transaction information storage service including, receiving a first group of one or more usage requests and a second group of one or more other usage requests, the usage requests of the first group each requesting an operation that includes storing indicated information about one or more transactions between multiple users involving at least one transfer of funds between those multiple users, the usage requests of the second group each requesting an operation that includes retrieving stored information about at least one transactions between users involving a transfer of funds between those users, the usage requests of the first and second groups being received from one or more remote programs via invocation of at least one interface provided for the transaction information storage service; for each of the received usage requests of the first group, responding by selecting one of the multiple logical storage partitions in which to store the indicated information for the usage request, the selected one logical storage partition for the usage request being specific to one of the multiple users involved in the at least one transfer of funds for the one or more transactions of the usage request; identifying a group of multiple mirror computing devices that are associated with the selected logical storage partition; and attempting to store a copy of the indicated information for the usage request on each of the mirror computing devices of the identified group; and for each of the received usage requests of the second group, responding by identifying at least one of multiple computing devices on which the stored information for the usage request is stored and providing that stored information from one of the identified computing devices.