Lost receipts
I wouldn't worry about it too much. It's a supporting document that will definitely come in handy if you ever go to your local DO to make info pass appointments because they would like to see all the notices that you've received from USCIS. Secondly if you have to consult with a lawyer regarding your case/file, he or she would like to see all the receipts/notices sent to you by USCIS.
On the other hand, if you just sit tight and wait for your interview letter, those prior notices have no meaning. When you are called for the interview, you will just take that last notice (interview letter) to appear for your interview, because USCIS has all your updated records, I mean your receipt which says your priority date and when your finger prints were completed.
If I were you, I would call up USCIS customer serivice line (1-800-375-5283) and request them if they can mail you a copy of your notices, because somehow you can't find them. Don't tell them that you have lost them because that may create further confusion and you definitely don't want your process to get delayed.