Where you are selling subscriptions, you will want to force the users to register / login before they can checkout. See the "How to apply Login/Register functionality to your Checkout Process" tutorial in the how to section of the eCart support page for information on this.
The user registration will be the user details.
In the orders table, you should have an OrderUserID column. This will be used to relate the order to the logged in user.
The first part of the "How to create an Order History page" how to on the eCart support page goes into how to accomplish this.
The eCart support page is herre: