You may already be familiar with slice.com. Slice syncs with your webmail account to automatically find your email receipts for purchases from common online retailers such as Amazon. The app then organizes those receipts into an easy to browse online dashboard. One of the best features of Slice is the ability to automatically track shipments when you buy something online. If it misses on your purchases you can always forward any email receipt to the service for indexing.
Now, if you like the idea, it is made better by their Android app. Its not perfect but having quick access to all your purchases, merchant contact info (for customer service), and package tracking is really convenient within the app. Its much easier/faster than searching through your emails for the right receipt.